A Day in the Life - Veterinary Medicine PDF

34 Unit 1 Comparative Anatomy and Physiology VetBooks.ir A Day in the Life Routine Days Aren’t Always Routine... Today seemed predictable as far as my days go. Herd...

34 Unit 1 Comparative Anatomy and Physiology VetBooks.ir A Day in the Life Routine Days Aren’t Always Routine... Today seemed predictable as far as my days go. Herd Missy was crying in pain and not able to bear checks (reproductive exams and routine health main- weight on her injured leg when the farmer brought her tenance work) dominated the day. Many of our dairy to our office. My associate, Dr. Deppen, examined her. clients ask a veterinarian in our practice to visit once It was obvious that the leg was very swollen and likely or twice a month to perform a herd check. When broken. Dr. Deppen took a radiograph of Missy’s tibia, called for a herd check, I examine cows for pregnancy a bone in the lower leg. (veterinarians can detect pregnancy in cattle bred for It was apparent from the radiograph that the tibia 30 days or more) and also make sure they have recov- had been broken into several pieces (Figure 3–2). We ered from calving. Keep in mind that dairy cows experi- offered to refer Missy to an orthopedic surgeon, a vet- ence a lactation peak shortly after calving. Therefore, erinarian who specializes in surgery of the bones. Know- farmers attempt to have their cows calve every 12 to ing a referral of this type can be quite expensive, the 13 months to maximize milk production. I help them owners wondered if we could repair the bone. accomplish this goal. Dr. Deppen and I discussed the options. We both I heard a familiar bark when I pulled into my felt that, considering the severity of the fracture, a cast second call of the day (another herd check). Missy, a or splint was not likely to be successful. Conversely, 120-pound Saint Bernard, let everyone know that I had we could attempt to perform the needed surgery. arrived (Figure 3–1). I first met Missy after she was I called the farmers to offer the choices at hand. I first injured while working on the farm. Her owner had driven told them that I am not an orthopedic specialist and his pickup to cut firewood. Dutiful Missy rode along in that the fracture was quite severe. Then I explained that the bed of the truck. When the truck stopped, Missy I could attempt the surgery. They agreed to allow me to jumped off, just as she had done many times before. perform the surgery, knowing that the operation might Unfortunately, this time Missy’s right hind leg became not be successful. I obviously needed a thorough knowl- caught and she fell. edge of bones before I could repair them. Photograph courtesy of Fred Brenner © 2017 Cengage Learning® FIGURE 3–1 Missy enjoying life at home. FIGURE 3–2 Radiograph of Missy’s fractured leg. MUSCULOSKELETAL SYSTEM FUNCTIONS The most visible function of bones is structure. The col- lection of bones in the animal forms the skeleton. This Objective provides the framework that defines an animal’s shape Describe the Functions of the Musculoskeletal System and size. Differences in both size and shape are very obvious in veterinary medicine. The skull provides a Bones furnish five basic functions: structure, leverage, clear example of this variation. When seeing only the protection, mineral reserves, and blood cell production. bones of the skull, it is easy to distinguish the skull of a Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  Chapter 3 The Musculoskeletal System 35 cat from that of a horse. Having muscles closely associ- Articular VetBooks.ir ated with the skeleton provides movement of the bones Cartilage Proximal Epiphysis at a joint. The movement of bones allows locomotion Bone Physis and function of the animal. Marrow Metaphysis The strength of bones also protects more fragile tissues. The rib cage gives protection to the heart and Cancellous or lungs, whereas the skull protects the delicate brain. Spongy Bone Bone acts as a reservoir for calcium and phosphorus. In times of need, the minerals are moved from the bone Medullary Cavity and sent into the bloodstream. Excess minerals can be stored in the bone. Calcium plays an essential role in muscle contraction and enzyme activity. Phosphorus is Artery necessary for energy metabolism within the cell. Bone, in response to several hormones, maintains a tight A Compact Diaphysis regulation on the blood level of these minerals. These Bone Tissue hormones, calcitonin and parathyroid hormone, will be Endosteum discussed in much greater detail in Chapter 10. The long bones are present in the legs (and arms in humans). The femur and humerus are classified as long bones. They have a dense outer shell and a hollow shaft. Bone marrow is made in this hollow center, the medul- Periosteum lary cavity. Bone marrow in turn produces blood cells. © 2017 Cengage Learning® Metaphysis Physis BONE STRUCTURE Distal Epiphysis Objective Detail the Structure of Bone Splitting a long bone along its length shows the typ- ical structure of bone (Figure 3–3). The outer shell is composed of dense or compact bone. The term cortical bone is also used for this region. The greater the forces placed on a bone, the thicker this layer will be. In the femur, this compact bone is thickest in the middle of the shaft, where greatest strain occurs. Within compact bone lies a more loosely arranged bone, called spongy or cancellous bone. Spongy bone B is found within the long bones but not inside the flat bones of the skull or pelvis. It only fills the ends of these long bones. Spongy bone is made up of tiny spicules and plates of bone. The spicules look random © 2017 Cengage Learning® but are actually arranged to maximize strength. The spongy arrangement keeps the weight of the bones much lighter than that of a solid bone of the same di- mension. The medullary cavity is located in the hollow center of the shaft. The bone marrow lies within the medullary cavity and the spaces of the spongy bone. As FIGURE 3–3 A. Illustration of bone structure. B. Photograph of the internal mentioned earlier, bone marrow produces blood cells. structure of bone. Bones are covered with a thin connective tissue called the periosteum. The periosteum blends into tendons and ligaments, binding them to the bone. The provides protection as the bones move against one periosteum has an extensive blood and nerve supply. another within a joint. The open spaces within bone Hence trauma to the periosteum is quite painful. The are covered with a similar connective tissue, the en- portion of bone within the joint is covered with car- dosteum. Both the periosteum and endosteum provide tilage and not by periosteum. This articular cartilage cells necessary for the repair of damage. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  36 Unit 1 Comparative Anatomy and Physiology A dried bone is composed of about 70% inorganic Blood vessels, nerves, and lymphatics run through VetBooks.ir minerals and 30% organic components. The inorganic this canal. Bone is laid in concentric circles around this minerals have a high level of calcium and phosphorus. channel, forming cylinders. Within these layers lie the This is found as crystals of hydroxyapatite (3Ca3(PO4)2· osteocytes that maintain the bone matrix. Communi- Ca(OH)2). The organic portion contains collagen fibers, cation occurs between osteocytes through microscopic polysaccharides, and cells. The fibers provide a frame- canals called canaliculi. Many osteons are joined to work on which the hydroxyapatite crystals can be de- form the layers of bone. A fine connective tissue layer, posited. Whereas organic fibers give the bone a small the periosteum, covers the external surface of bone. amount of elasticity and resiliency, minerals give bone The periosteum is the source of blood vessels that its typical hardness and strength. Without the organic supply nutrients to the bone tissue. The small vessels material bones would be quite brittle. The collagen enter the bone through Volkmann’s canals, which cross fibers and the hydroxyapatite crystals make up the osteons. Larger vessels may enter the shaft of a bone matrix that surrounds the cells. through a nutrient foramen. Typically these vessels There are three types of bone cells. Osteoblasts, found supply the bone marrow. It is important to recognize in close association with the periosteum, lay down the col- that a nutrient foramen could be mistaken for a crack in lagen matrix. Osteoblasts are important in increasing the one cortex of a bone when seen on a radiograph. diameter of bone and in the healing of fractures. During the formation of bone these osteoblasts become encased in matrix, developing into osteocytes. The osteocytes are found in microscopic cavities of mature bone, the lacu- JOINT TYPES AND MOVEMENTS nae. The osteocytes are responsible for maintaining the bone matrix. Osteoclasts are large multinucleated cells Objective that release the minerals from bone. Bone is a living tissue Name Joint Types and Their Accompanying that is always being remodeled in response to physical Roles in Movement forces on the body and the body’s need for calcium. Joints form where other tissues join two bones. Osteoporosis is a condition in which the bones Generally joints are classified by the amounts or types lose their normal density. Several disease conditions of movement allowed (Figure 3–5). can result in this decrease of bone mass. The problem Fibrous Joints: The bones in a fibrous joint are can also occur in animals when a limb is not used for brought together with a dense connective tissue. These long periods (e.g., following extended time in a cast). are also called fixed joints because little movement is Because the bone is not subjected to physical forces, possible. This type of joint can be found in the skull, new mineral is not deposited. Osteoclasts continue to where it is called a suture. release the minerals into the bloodstream. The bones Cartilage Joints: As the name suggests, the bones can become so thin that they can break under normal in this joint type are connected with cartilage. The usage. Osteoporosis that occurs from disuse is revers- growth plate of young animals serves as an example. ible once the animal begins to use the leg. The growth plate exists within a bone and allows for Bone is composed of a collection of microscopic units called osteons or haversian systems. There is a rapid growth. The cartilage layer within the growth haversian canal in the center of the osteon (Figure 3–4). plate is eventually replaced by bone as the animal reaches adulthood. A symphysis is another type of car- tilage joint. Symphysis is found, for example, between the halves of the pelvis as well as between those of the lower jaw. A cartilage joint also occurs between two vertebrae and includes the intervertebral disk. Synovial Joints: These joints are the true moveable Photomicrograph courtesy William J. Bacha, PhD, joints. A dense layer of bone at the joint is covered with a layer of cartilage. This articular cartilage cov- ers the contact surfaces of the two bones (Figure 3–6). The joint is enclosed with a joint capsule. The outer and Linda M. Bacha, MS, VMD. layer of the joint capsule contains strong connective tissue. This is lined with a synovial membrane, which produces synovial fluid. Synovial fluid provides lubri- cation to the joint and carries nutrients to the cartilage. Ligaments are also present to provide strength to the FIGURE 3–4 Microscopic structure of bone, showing osteons with a joint. Ligaments are a dense fibrous connective tissue central canal. The bone matrix is deposited in a circular arrangement. band that connects bone to bone. Tendons, connect- Darkly stained osteocytes are visible within the matrix. ing muscle to bone, also can increase the stability of a Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  Chapter 3 The Musculoskeletal System 37 Synovial Joints Fibrous Joint VetBooks.ir Suture Pivot Cartilage Joint © 2017 Cengage Learning® Ball and Socket Hinge Symphysis FIGURE 3–5 Joint types. joint. Certain joints have a meniscus, which is a hard describes a movement in which the end of the limb is cartilage pad. The meniscus acts as a cushion between moved in a circular motion. the bone ends. The knee joint has ligaments both inside The type of motion allowed also describes specific and outside the joint capsule. Menisci are also present types of joints. The simplest, the hinge joint, allows as cushions for the wide range of motion in this joint. movement in one axis. The classic example of a hinge Several terms are used to describe the motion joint is the elbow. The arm can be flexed and extended within a joint. The same description can be used for the at the elbow in only one plane. A pivot joint allows muscle group that causes that motion. Flexion occurs rotation around a point. The pivot joint between the when the angle between the two bones gets smaller. first and second vertebrae allows the head to rotate. The opposite motion, extension, occurs as the angle The wrist is an example of an ellipsoid joint, which between the bones increases. Abduction occurs when allows motion not only in hinge fashion but also in a part is moved away from the body, and adduction as rotation. The ball-and-socket joint, such as the hip or the part is moved closer. When a part spins on its long shoulder, allows motion in any direction. This type of axis, the movement is termed rotation. Circumduction joint provides the greatest variety of motion. Fat Pad AXIAL AND APPENDICULAR SKELETONS Objective List the Two Major Sections of the Skeleton, Name the Corresponding Bones, and Compare Species Differentiation Tremendous similarities exist in the skeletal structure of the domestic species (Figure 3–7). The skeleton can be divided into two major sections. The axial skeleton contains the skull, vertebrae, ribs, and sternum. The appendicular skeleton consists of the bones of the limbs. © 2017 Cengage Learning® The total number of bones varies between species and even within a species. For example, dogs may or may not be born without the first digit (dewclaw) on their legs. Moreover, some dogs have very short tails with a small number of vertebrae, whereas others have long tails and more vertebrae. A typical dog has about 320 bones (134 in Articular Cartilage Cruciate the axial skeleton and 186 in the appendicular skeleton). of Femur Ligament On the other hand, horses have fewer bones in the distal FIGURE 3–6 Internal structure of the knee joint. Probe identifies a meniscus. limbs than dogs, with a total of only 205 bones. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  38 Unit 1 Comparative Anatomy and Physiology Gliding Joints VetBooks.ir (5) Sacral Vertebrae (15–21) Coccygeal (6) (18) (7) or Caudal Lumbar Thoracic Cervical Vertebrae Vertebrae Vertebrae Vertebrae Skull Atlas (C1) Axis (C2) Pelvis Mandible Ball and (Lower Jaw) Socket A Joint Femur Scapula Patella a Humerus Fibula Ulna Ribs Sternum Radius Tibia (Xiphoid Tarsus Cartilage) Carpus (Hock) Olecranon (Carpal Bones) II or IV Metatarsal II or IV Metacarpal P1 or Long (Splint) III Metatarsal Pastern Bone or Proximal III Metacarpal (Cannon Bone) (Cannon Bone) © 2017 Cengage Learning® Phalanx Hinge Joint P2 or Short Pastern Bone or Intermediate Phalanx P3 or Coffin Bone or Distal Phalanx (3) Sacral Vertebrae (14–23) (7) (13) (7) Coccygeal Cervical Thoracic Lumbar (Caudal) Vertebrae Skull (Cranium) Vertebrae Vertebrae Vertebrae (“Tail T Bones”) Ribs Pelvis Mandible d Femur Hyoid B Clavicle Scapula Sternumm Humerus Patella “Elbow” (“Kneecaps”) Radius Ulna Fibula Tibia Carpals © 2017 Cengage Learning® Digits Metacarpals Tarsals Metatarsals r (Hock) Phalanges FIGURE 3–7 Comparison of the skeletons of a horse and a cat. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  Chapter 3 The Musculoskeletal System 39 Temporal VetBooks.ir Temporal Parietal Frontal Occipital Frontal Nasal Zygomatic Arch Sphenoid Nasal © 2017 Cengage Learning® Maxilla Mandible Incisive Front View Side View FIGURE 3–8 Skull structure of a horse. Each bone is given a distinct name. In addition, consider the long narrow nose of the collie and the short the parts of the bone are also named. These sites often broad nose of the pug. Regardless of appearance, the serve as points of muscle attachments or form portions basic skull anatomy remains quite similar. The size of of joints. Identifying regions of the bone gives more the bones in the skull accounts for differentiation. detailed information about the particular bone. For The vertebral column extends through the length example, before operating on Missy’s leg, a review of of the body from the skull. The vertebral column pro- the surgical procedure was necessary. This protocol tects the spinal cord and allows movement. Force from mentioned several points of the tibia that would act as the hind limbs transfers through the spinal column to landmarks during the surgery. propel an animal. Found in the axial skeleton, the skull (cranium) The body of the vertebra is covered with a bony combines numerous flat bones (approximately 50 in the arch, creating the vertebral foramen (Figure 3–10). The dog) (Figure 3–8). The skull performs many functions, entire series of vertebrae with all of the bony arches most notably protection for the brain and other organs creates the vertebral canal, housing the spinal cord. of the special senses (sight, hearing, taste, and smell). A The spinous process and transverse processes are sites moveable mandible (lower jaw) allows animals to secure of attachment for tendons and ligaments. The length and chew food. The jaw joint is a synovial joint, whereas of the spinous process varies between species and other bones of the skull are connected with the tight between the different regions of the vertebrae. The fibrous joint called a suture. The bones of the jaws hold vertebrae in the thorax of the horse have very long the teeth, which vary tremendously among species due spinous processes creating the region called the withers. to diet adaptations (Figure 3–9). The shape of the skull Between the bodies of the vertebrae (except between C1 also differs among and within species. For instance, and C2 and within the sacrum) lie intervertebral disks. Skull (Cranium) Spinous Process Arch Lamina Lamina Vertebral Foramen © 2017 Cengage Learning® Foramen © 2017 Cengage Learning® Transverse Process Mandible Vertebral Body FIGURE 3–9 Radiograph of the skull of a dog. FIGURE 3–10 The structure of a vertebra. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  40 Unit 1 Comparative Anatomy and Physiology The disks have strong, fibrous outer rings and soft, VetBooks.ir spongy centers. Disks provide cushioning between the Spinous vertebrae bones. The arrangement of vertebrae allows Process for movement and bending in any direction and also allows for a certain amount of twisting along the long Transverse Process axis of the body. The intervertebral disk has a clinical significance in many dogs. Although intervertebral disk disease can oc- cur in any breed, certain breeds, such as the dachshund, Pekingese, and cocker spaniel, have higher incidences of the disease. In this condition, the center of the disk becomes less spongy. Pressure between the vertebrae causes the center to rupture through the fibrous outer layer. The bulging disk can press on nerves or the spinal Body cord. Dogs with this condition may experience severe pain and can be paralyzed. Depending on the severity, Thorocic Vertebra the afflicted dogs may be treated medically or surgically. The vertebral column is broken down into ana- tomic divisions. At the head end, the cervical vertebrae make up the neck (Figure 3–11). Mammals, from cats to horses to giraffes, have seven cervical vertebrae. The Spinous first cervical vertebra is also called the atlas. This verte- Process bra possesses a unique shape and allows up-and-down motion of the head, as in nodding to signify yes. The axis, the second vertebra, permits the head to rotate back and forth, as seen in shaking the head to indicate no. The remaining five cervical vertebrae are all similar in size and shape. The thoracic vertebrae are found in the next divi- Body sion of the spinal column. There is one set of ribs for Transverse each thoracic vertebra (Figure 3–12). The ribs are flat Process bones (like the bones of the skull) and are essential to protect the heart and lungs. This framework also al- Lumbar Vertebra lows expansion of the lungs that occurs during breath- ing. Dogs, cats, cattle, goats, and sheep all have 13 pairs of ribs. Horses have 18 thoracic vertebrae. The sternum, a group of bones, forms the floor of the thorax. In older animals, the cartilage joining the ribs to the sternum can be replaced with bone. The dog has nine sternal Wing ribs that join to the sternum with costal cartilage. Cervical Vertebrae © 2017 Cengage Learning® Spine of Sacrum, Ventral View Scapula FIGURE 3-12 Comparison of the structure of thoracic, lumbar, and sacral vertebrae. © 2017 Cengage Learning® The lumbar vertebrae, located in the lower back, lie between the thoracic vertebrae and the pelvis Shoulder Joint (Figure 3–13). This area flexes and extends as animals Humerus walk and run. In addition, these vertebrae support the FIGURE 3–11 Radiograph of the cervical spine of a dog. organs in the abdomen. The muscles of the abdomen  Chapter 3 The Musculoskeletal System 41 Thoracic Vertebrae Lumbar Vertebrae a species. The typical dog has 20 caudal vertebrae, but VetBooks.ir this can range from 6 to 23. The appendicular skeleton includes the bones of the forelimbs and hind limbs. A study of this part of the skeleton provides a clear examination of compar- ative anatomy. Although the same anatomic terms are used for all mammals, great differences exist in the numbers and sizes of bones in the mammalian appen- dicular skeleton. For instance, a dog has four or five © 2017 Cengage Learning® toes, whereas a horse has only one. The forelimb, or thoracic limb, does not have a bony connection to the axial skeleton. The scapula, or shoul- der blade, lies flat against the rib cage (Figure 3–15). The scapula connects to the axial skeleton with a group of muscles. This attachment allows the scapula to move Sternum Rib over the rib cage. This rotation ranges as high as FIGURE 3–13 Radiograph of a cat, showing the thoracic and lumbar 25 degrees in animals such as cats while running. This spine. Ribs and sternum are also visible. flexibility is also useful in cats as they land after a jump. As the cat falls, it extends its front legs fully at both the attach to these vertebrae, forming a sling that supports scapula and the elbow. As the front feet hit the ground, internal organs. the elbow flexes and the scapula rotates. The cat makes The sacrum, a group of three sacral vertebrae, this very coordinated act look quite graceful. Clinically, fuses to support the pelvis (Figure 3–14). In addition, this is of significance when cats fall from extreme the sacrum articulates with the last lumbar vertebra heights. In large cities, this happens often as cats tumble and the first caudal vertebra. The sacrum then joins from balconies or windows of tall buildings. In high-rise with the pelvis, allowing the hind limbs to support the syndrome, the falling cat rarely breaks a leg; however, weight of the body. This connection can be damaged. it will often break its lower jaw. The high speed of the The pelvis may split away from the sacrum when falling cat forces the jaw to contact the ground. dogs and cats are hit by cars (HBC). During this type of accident, fracture of the pelvis itself is also com- mon. Very painful lameness often results from a split pelvis or pelvic fracture. Many of these fractures heal if the animal’s activities are restricted. In severe cases, surgeries may be required. The final group of vertebrae is called caudal. These small vertebrae comprise the tail. As mentioned, the numbers of vertebrae vary among species and within Lumbar Vertebrae Pelvis Spine © 2017 Cengage Learning® © 2017 Cengage Learning® FIGURE 3–14 Radiograph of the lumbar spine of a dog. A portion of the pelvis is also visible. This dog is showing an age-related change called spondylosis. In spondylosis, bone spurs are formed that can eventually bridge between vertebrae. FIGURE 3–15 The scapula. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  42 Unit 1 Comparative Anatomy and Physiology Spine of Scapula Radius VetBooks.ir Growth Plate © 2017 Cengage Learning® Carpal Shoulder Bones Joint Metacarpals Ulna Humerus FIGURE 3–17 Radiograph of the radius and ulna of a young dog. Note the growth plates visible in the radius and ulna. © 2017 Cengage Learning® Table 3–1 Number of Carpal Bones © 2017 Cengage Learning® Dog 7 Ruminants 6 FIGURE 3–16 Radiograph of the scapula and shoulder joint of a dog. Horse 7 or 8 The humerus is the upper bone of the forelimb. The scapula joins the humerus through a shallow two smaller metacarpal bones, also called the splint ball-and-socket joint that allows for a wide range of bones. These splint bones are tightly attached to the motion (Figure 3–16). The humerus then joins at the cannon bone with a ligament. In young horses during elbow with the radius and ulna. The ulna runs to the training, the periosteum in this region can become point of the elbow, where a groove accepts the end inflamed and produce lameness. Ruminants such as of the humerus. The olecranon is the portion of the cattle and sheep also have one very large metacarpal ulna that makes up the point of the elbow. This is a consistent region in all species. In dogs and cats, the ulna extends distally as far as the radius. In horses, Carpal Bones Metacarpals however, the ulna fuses to the middle of the radius and the distal portion is absent. The radius closely attaches to the ulna and forms the remainder of the elbow joint (a hinge joint that permits motion in only one plane). The forearm can be rotated, but this occurs between the radius and ulna, not at the elbow joint. The radius and ulna run to the level of the carpus (Figure 3–17). The carpus in animals corresponds to the wrist in humans. The carpus, a group of bones, is arranged in two rows. Table 3–1 lists the number of carpal bones found in several species. The carpal bones © 2017 Cengage Learning® join to the long metacarpal bones. In this region, dif- ferences among species become very dramatic. Dogs and cats have four long metacarpal bones and one much smaller. The smaller bone associates with the first digit, called the dewclaw (Figure 3–18). As previ- ously stated, horses have only one major metacarpal bone, which corresponds to the third one in other Phalanges species. This large weight-bearing metacarpal bone FIGURE 3-18 Radiograph of the carpus, metacarpals, and phalanges of is often referred to as the cannon bone. The horse has a dog. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  Chapter 3 The Musculoskeletal System 43 Sacrum VetBooks.ir Phalanx 1 P1 Pelvis Phalanx 2 P2 © 2017 Cengage Learning® Hip Joint Phalanx 3 P3 FIGURE 3–19 Radiograph of the foot of a horse. Femur © 2017 Cengage Learning® bone. As the ruminant embryo develops, the third and fourth metacarpal bones fuse into one structure. The toes or digits contain three bones, the phalan- ges. The number of toes corresponds to the number of metacarpal bones. The singular form of the word FIGURE 3–20 Radiograph of the pelvis. phalanges is phalanx. The last phalanx is covered by the nail or hoof (Figure 3–19). The sesamoid bones are quite small and are found in tendons where a change in direction occurs. Sesamoid bones help minimize wear on the tendon. In horses, there are two sesamoid bones The ilium, ischium, and pubis meet to form the at the joint between the metacarpal bone and the first acetabulum. The acetabulum, or socket portion of the phalanx. In horses, this joint is often called the fetlock hip joint, lies on either side of the pelvis. This socket joint, and the region between the fetlock and hoof is the accepts the ball portion of the femur. The femoral head pastern. Another sesamoid, the navicular bone, occurs is held in place by a strong ligament within the acetab- at the joint between the last two phalanges. ulum and by the surrounding muscles. The hip joint In contrast to the thoracic limb, the pelvic limb serves as a classic example of a ball-and-socket joint. forms a bony connection with the spine. As discussed The femur extends down the leg to the level of the earlier, the fused vertebrae of the sacrum join with the knee or stifle. At this point, the femur joins with the pelvis (Figure 3–20). The pelvis is made of two halves. tibia (Figure 3–22). Also present in the lower leg is a Each half divides into regions named the ilium, ischium, fibula. This bone is much smaller than the tibia and and pubis. The ilium, the most forward portion of the plays a less significant role in our domestic species. The pelvis, joins with the sacrum at the iliosacral joint. The complicated knee joint is supported by two external ilium provides a large flat surface for the attachment of ligaments, the medial and the lateral collateral liga- muscles. The craniodorsal portion of the ilium is often ment, and also by the tendon of the patella, or kneecap. visible in lean cattle and is known as the point of the The patella is the largest example of a sesamoid bone. hip or the hook bone (Figure 3–21). The ischium makes Internally, two ligaments located within the knee up the caudal portion of the floor of the pelvis, whereas cross in an X (Figure 3–23). These cruciate ligaments the pubis forms the cranial floor. The caudal portion provide great stability to the joint but can be damaged of the ischium is visible in lean cattle on either side of and torn. A common cause of lameness in dogs is a torn the tail. In lay terms these are called the pin bones. The cranial cruciate ligament. Whereas this tear typically halves of the pelvis join in the middle, forming the pubic occurs in humans secondary to trauma, it is a degener- symphysis, a basically immovable cartilage joint. During ative condition in dogs. The ligament deteriorates with parturition or birth the cartilage softens in response age and finally tears. This is important because dogs to hormonal changes, thus permitting the newborn to often develop the problem in both knees. Veterinarians move through the pelvic canal. diagnose this problem by pressing back on the end of  44 Unit 1 Comparative Anatomy and Physiology VetBooks.ir A © 2017 Cengage Learning® B © 2017 Cengage Learning® FIGURE 3–21 A. Photograph of the rump of a cow. Notice the wings of the ilium (the hooks) and the ischium on each side of the tail (pins). B. Photograph of the pelvic bones of a cow. Notice the wings of the ilium and the ischium. Compare to the position on the cow. the femur and forward on the tibia. If the ligament is leg to the hock joint (which corresponds to the ankle in torn, the tibia will slide forward, much more than in a humans). There, a hinge joint between the tibia and tar- normal leg. This cranial drawer sign indicates a diag- sal bones exists. The many tarsal bones are arranged in nosis of torn cruciate ligament. In small-breed dogs, much the same fashion as the carpal bones of the front this condition often shows significant improvement leg. (It is a common mistake for students to confuse the with restricted activity. In larger dogs, surgery is often legs associated with carpal or tarsal bones. It is helpful required to correct the condition. to remember t, as in toes and tarsal, which are both The tibia, the heavy bone of the lower leg, closely associated with the hind limb.) attaches to the much smaller fibula. The tibia is the In the hind foot, the metatarsal bones and phalan- main weight-bearing bone of this region. The fibula ges are arranged identically to the front foot. The num- supports little weight but does provide for muscle ber of bones in the hind foot generally matches that of attachments in that region. The tibia extends down the the thoracic limb.  Chapter 3 The Musculoskeletal System 45 Femur Fibula In young animals, the cartilage growth plate contin- VetBooks.ir ues to grow. At the same time, at the edge, osteoblasts move into the cartilage. The bones increase in length as the osteoblasts lay down more osteoid matrix. The growth is under the control of growth hormone and the sex hormones. In this process, called ossification, the cartilage is replaced by bone tissue. Ossification involves more than just minerals being deposited. The cartilage is replaced by true bone, with all the appro- priate cells, structure, and blood supply. As maturity approaches, the cartilage grows more slowly, and the © 2017 Cengage Learning® growth plate narrows. Eventually, bone completely re- places the growth plate. After the growth plates close, bones do not increase in length. For proper development to occur, the rate of limb growth must match. Injury to a young animal may Patella Tibia Tarsal result in premature closure of a growth plate (the Bones weakest site in the bone). With sufficient trauma, the FIGURE 3–22 Radiograph of the hind limb, showing the femur and tibia. bone fractures at the growth plate. As this injury heals, the growth plate may be replaced with bone, caus- ing the wounded site to cease growth in length. As a Cruciate Ligaments result, the limb on the injured side is shorter. However, Fat pad animals with mismatched limbs compensate very well by adjusting the positioning of the shorter leg. For example, if one pelvic limb is shorter, the hock and stifle are kept in greater extension. A significant problem can result when damage oc- curs to either the radius or ulna. If only one bone in the forelimb stops growing, the increasing length of the other forces the leg to curve. This problem can be a result of trauma but can naturally occur in certain breeds of dogs. Basset hounds typically have premature closure of the distal growth plate of the ulna. The radius is bound © 2017 Cengage Learning® between the humerus and the carpus. Because it grows faster than the ulna, it develops a curve to fit between the other bones (Figure 3–24). In most of these dogs, the only effect is the typical short-curved leg. In certain Femur Humerus Radius FIGURE 3–23 Photograph of the internal structure of the knee. Note the cross pattern of the two cruciate ligaments identified by the probe. BONE GROWTH AND REMODELING Objective Explain How Bone Grows and Remodels © 2017 Cengage Learning® Obviously there must be a mechanism for an imma- ture animal’s bones to grow longer. A long bone, such as the femur, divides into the shaft, or diaphysis, and the ends, or epiphyses. In a young animal, a cartilage plate separates the diaphysis from the epiphysis. These Ulna growth plates, or epiphyseal plates, are the sites of FIGURE 3–24 Radiograph of the radius and ulna of a basset hound. The elongation. Bones also increase in diameter, with new growth plate of the ulna closed prematurely, forcing the radius to curve cells deposited under the periosteum. as it continued to grow. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  46 Unit 1 Comparative Anatomy and Physiology animals, the pressure of the radius partially dislocates VetBooks.ir (subluxates) the humerus from the elbow joint. This can cause such severe lameness that surgical intervention may be necessary. Radiographs are produced when a stream of x-rays is passed through a body part, causing exposure of a piece of photographic film. X-rays are a form of elec- tromagnetic radiation that can pass through living tissue. (The term x-ray is commonly used to describe the resulting picture. However, it is technically correct to refer to the photograph as the radiograph. Invisible x-rays expose the film.) Dense structures allow only an extremely small amount of x-rays to pass, and are termed radiopaque. Tissues, such as teeth and bone, fit into this category and appear light on the radiograph. (Refer to Missy’s radiograph, Figure 3–2, at the beginning of the chapter to see how white the bone appears.) Radiolucent tis- sues allow much more of the x-ray energy to pass through and show up dark on the radiograph. Varia- tion among tissues permits the radiograph to be inter- preted. The fracture in Missy’s tibia shows up as a dark © 2017 Cengage Learning® line through the center of the white bone. Many radiographs are shown in this text. In these radiographs, there are basically five stages of density. Ranging from the most radiolucent to the most radi- opaque are: (1) air (such as in the lung), (2) fat, (3) soft tissue or muscle, (4) bone, and (5) mineral (such as in FIGURE 3–25 Radiograph machine. teeth). Remember, air is the darkest and mineral the whitest on a radiograph. X-rays are capable of damaging living tissues when easy transfer of the image to a specialty clinic or to a used at high dosages and if repeated time and again. radiologist for consultation. Radiology, the study of radiographs, is an essential Radiographs throughout this text show anatomy part of veterinary and human medicine. However, the in both healthy and diseased animals. Keep in mind application of radiology must be done with judgment that radiographs show a two-dimensional picture of a to minimize exposure of the animal, and the human three-dimensional object. Although not always shown doing the procedure (Figure 3–25). Film and the cas- in the text, standard procedure mandates that two settes that carry films are designed for minimum x-ray views (at 90 degrees to each other) of a single body part use. Also, specially designed lead aprons and gloves should be radiographed. Two views give the veterinar- are used to minimize exposure to humans. Lead ef- ian a better understanding of the structure in question. fectively prevents x-ray penetration. To ensure safe working conditions, technicians and veterinarians are monitored for their x-ray exposure level. RELATION OF BONES, MUSCLES, In photography, there has been a dramatic shift AND MOVEMENT away from cameras containing film to digital cameras. Radiology equipment is going through the same tran- Objective sition. Digital radiology equipment converts the x-rays Relate Bone and Muscle Groups to Movement into an electrical signal that in turn is converted into an image on a computer screen. This type of technol- Muscles are included in this chapter because of their ogy is expensive but offers many advantages over the close association with bone. Together, bones and mus- old film method. The radiographic image is available cles provide the ability to move. Skeletal muscles attach much more quickly, and the need for radiographic to bone or cartilage by connective tissue. Each level film, developing equipment, and chemicals is elimi- of muscle is covered in connective tissue, which com- nated. Having the image on the computer also allows bines to form a tendon. In most locations, this tendon for alterations in brightness, contrast, and size. Having appears as a narrow cord. Some muscles end so close the radiographic image as a computer file also allows to the bone that no obvious tendon can be seen, or they Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  Chapter 3 The Musculoskeletal System 47 attach with a broad sheet of connective tissue called an Fortunately, animals (including humans) do not have to VetBooks.ir aponeurosis. Muscles are described by the location of think about the individual functions of all these muscles. their attachments. The more fixed point is called the These muscles can be controlled voluntarily as well. origin. The more moveable point is called the insertion. Muscles are often arranged in groups to achieve a Muscles of the limbs always have the most distal point single function. For example, three muscles make up as the insertion. For example, the biceps (the muscle on the hamstrings (biceps femoris, semimembranosus, and the front of the upper arm) originates on the scapula semitendinosus). These muscles work together to extend and inserts on the radius and ulna. the hip and flex the knee (Figure 3–26). The muscle or Muscles that cross a joint are also described on the group working together to achieve the desired move- basis of the type of motion that they cause. Extensors ment is called the agonist. In each situation, there is an cause the bones to move into straighter alignment antagonist muscle or group that performs the opposite or open the joint. Flexors on the opposing side bend movement. For example, when the hamstrings are the the joint or decrease the angle between the bones. agonist, the quadriceps muscles on the front of the leg The names may also describe the muscle’s shape, its act as the antagonist. The antagonist naturally returns the location, or the number of heads. body part to its prior position. A less obvious function Although skeletal muscle is described as volun- is that the antagonist acts to make the agonists function tary, much of its activity is controlled without con- much more smoothly by providing resistance to the ago- scious thought. Muscles that control balance, posture, nist. Finally, the agonist and antagonist can contract with breathing, and swallowing often function spontaneously. equal force to stabilize a joint without movement. Sartorius Biceps A Femoris © 2017 Cengage Learning® Vastus Lateralis Vastus Lateralis Femur B Sciatic Nerve Semitendinosus © 2017 Cengage Learning® Cut body of biceps femoris FIGURE 3–26 A. Superficial muscles of the thigh. B. Deep muscles of the thigh. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  48 Unit 1 Comparative Anatomy and Physiology CLINICAL PRACTICE VetBooks.ir Objective Tibia Connect the Academic Materials Pertaining to the Musculoskeletal System to Clinical Practice As in Missy, the Saint Bernard, fractured bones are a com- mon abnormality. If the bone is broken into two pieces, a Intramedullary clean break, it is called a simple fracture. A comminuted Pin fracture results in several fragments of bone. Missy’s ra- diograph showed that she had a comminuted fracture. A compound (open) fracture results when one of the bone ends punctures through the skin. The risk of bone infec- tion is higher in compound fractures. For a fracture to heal, the bone ends must be put back in alignment and held without movement. Just as in humans, many fractures can be repaired with a cast Cerclage or splint. In veterinary medicine, the support must be Wire © 2017 Cengage Learning® made to hold the weight of the animal. After correc- tion, the animal must be able to use the leg. Very active pets have a hard time keeping casts in place, let alone clean and dry. Surgical correction is often used to repair fractures. With surgery, some form of surgical stainless steel, FIGURE 3–28 Radiograph showing the repair of Missy’s tibia. The bone such as a bone plate, is used to support the bone. A was repaired with an intramedullary pin and six cerclage wires. bone plate is applied to the outside edge of the bone and attached with screws (Figure 3–27). We offered to refer Missy for this type of surgery. Bone plates offer a very stable form of correction. Proper size plates must bone. To stabilize the pieces, I also added several wires be correctly shaped to fit the bone. wrapped around the bone fragments. These are called Another method of repair is termed an intramedul- cerclage wires. Radiographs are taken to ensure proper lary pin. I used this type of correction on Missy. During placement of the pin before completing the surgery. the surgery, I drove a stainless steel pin into the center One possible mistake would have been to drive the of Missy’s broken tibia (Figure 3–28). Th