Myology - Book Part 1 PDF

## Chapter III. MYOLOGY Myology studies the aspect, structure and the role of muscles and their accessory structures represented through the tendons, the fasciae, the synovial bursae or the synovial sheaths. The muscles represent specialized structures for motion, being active organs by their prop...

## Chapter III. MYOLOGY Myology studies the aspect, structure and the role of muscles and their accessory structures represented through the tendons, the fasciae, the synovial bursae or the synovial sheaths. The muscles represent specialized structures for motion, being active organs by their proper contraction under the nerve stimulation. From a morphological point of view, and depending on the type of fibers, the muscles may be differentiated into the following types: * **Striated muscles** which are striped muscles; they are connected with the skeleton, and for this reason they are called the **skeletal muscles**. At both endings of the muscle, the muscle fibers continue with tendons. * **Smooth muscles**, that are nonstriated, being spindle-shaped cells which constitute the middle layer of the walls of the hollow organs, such as the intestines, the urinary bladder, the blood vessels, or they may also be found in several glands. This is why the smooth muscles are called the **visceral muscles**. Their contraction falls under the vegetative control, therefore not being controlled at will. * **Cardiac Muscles** form the myocardium and belong to a special type of fibers which have common characteristics with the smooth muscles (involuntary contraction) but they have cross striations and tendons similar to the striated muscles. The fibers of the cardiac muscles are typically branched to form a network. They are interconnected by intercalated discs giving that tissue the appearance of a syncytium. ### 3.1. THE STRUCTURE OF THE SKELETAL MUSCLES Generally, a skeletal muscle consists of fibers (myocytes) that are bundled into fascicles by the fibrous sheaths which are themselves grouped together to form the belly (Venter) of the muscle. The fibrous sheath supports the muscle's function both by a resisting passive stretching of the tissue and by distributing the forces applied to the muscle. Skeletal muscles are bounded by a strong layer of connective tissue called epimysium (Epimysium), which continues to the tendon as an epitenon (Fig. 3.1.). The epimysium anchors the muscle tissue to tendons at each end, where the epimysium becomes thicker and collagenous. It also protects muscles from friction against other muscles and bones. Within the epimysium there are multiple bundles called fascicles, each containing from 10 to 100 or more muscle fibers together, being surrounded by the perimysium (Perimysium). Besides surrounding each fascicle, the perimysium is a pathway for the nerves and the flow of blood within the muscle. The thread like muscle fibers are the individual muscle cells (myocytes) and each one is encased cells of the wodomia (Endomystum) of cells (myocyte). Within the cells of the muscle there are the myofibrils, which are bundles of protein filaments (Fig. 3. 1). The myofibrils are complex strands of several kinds of protein filaments organized together into repeating units called sarcomeres. The striated appearance of both the skeletal and the cardiac muscles results from the regular pattern of the sarcomeres within their cells. The filaments in a sarcomere are composed of actin and myosin. At both capitis of the skeletal muscle, the tendons (Tendo) attach the muscles to the bones, cartilages, fasciae, or at the bottom of the skin, etc, through the fibrous or cartilaginous entheses. The fibrous entheses are characterized by dense fibrous connective tissue attached to diaphyses of the long bones depending on whether the tendon inserts directly into a bone or periosteum respectively. This type of enthesis is found in muscles which insert on the crests, such as the deltoid muscle on the humeral crest, and the muscles attaching to the linea aspera of the femur, such as the adductors or quadriceps (proximal insertion) muscles. Fibrocartilaginous entheses are characterized by fibrocartilage at the tendon-bone interface and are typical of epiphyses and apophyses. These types of entheses are more common than fibrous entheses and are likely to cause overuse injuries such as those of the calcaneal tendons (Apostolakos et al., 2014). The tendons are usually thinner than the muscles. What it is known as the origin of the muscle is proximally placed to the insertion (or termination), the last one coresponding with the distal insertion. Usually, the muscles work changing the place of action, an aspect that is known as the reversible insertions. The tendons are noncontractile, only transferring their forces of movement towards the leverage arms, representated by bones. As for tendons, the muscles may have a cylindrical tendon or a large and extended band, where the fibers are parallelly placed as in the case of the aponeurosis (Aponeurosois). In the case of the spindle-like muscle, the following characteristics may be described (Fig. 3. 2): * **The head (Caput)**, which lies immediately below the origin of the muscle; * **The belly (Venter)**, which represents the middle of the muscle; * **The tail (Cauda)**, which is found at the distal part of the muscle. Depending on the numbers of heads, the muscles can be one-, two- (biceps), three- (triceps) or four- (quadriceps) headed. In the same way, some muscles have two bellies (M. digastricus) or many bellies (poligastricus) such as the m. rectus abdominis. A muscle can also have many tails (distal insertions), like the flexor and extensor muscles of the digits. Depending on the shape and the orientation of the fibers, the muscles may be (Fig. 3.2): * **Spindle-like muscles** (M. fusiformis); * **Sheet-like muscles** (M. planus); * **Ring-shaped muscles** (M. orbicularis) which circumscribe the natural openings, being called the **sphincter** muscles. * **Pennate muscles** (M. pennatus) - the muscle fibres join the tendon at an angle: * **Unipennate muscles** (M. unipennatus)- with two parallel sheets; * **Bipennate muscles** (M. bipennatus)- two tendon sheets of different directions; * **Multipennate muscles** (M. multipennatus) - several tendon sheets of different directions; * **Triangular muscles** (M. triangulatis); * **Quadrangular muscle** (M. quadratus), etc. Depending on the position of the muscles in relation to the joint's angle, the muscles that act under a joint may be divided into: * **The extensor muscles**, being placed outside of the flexion agle, * **The flexors**, which are placed in the flexion angle, * **The adductors**, represented by the muscles that bring a part of the body nearer to the median plane, * **The abductor muscles**, which act in opposition to the adductor muscles, * **The rotator muscles**, which rotate some parts of the skeleton and are placed behind or beside each other, form the * **Pronators**, * **Supinators**, * **The sphincter muscles**, * **The levator muscles**, * **The depressor muscles,** * **The tensor muscles**, etc. All muscles with similar action are called **agonist muscles**, acting in opposition to the **antagonist muscles**. ### THE ADNEXAE OF THE MUSCLES. THE ACCESSORY STRUCTURES 1. **The fascia** (Fascia) represents the regional connective tissue which forms the conjunctive membrane that separates the muscles from each other but, at the same time, brings the muscles together into a group. The thickness and the strength of a fascia vary from a region to the other and from the superficial (Fascia superficialis) to the deep fascia (Fascia profunda). The superficial fascia includes the cutaneous muscle (Mm. cutanei) in some regions into its blades. Sometimes, the muscles insert on the fascia. Some thickenings of the fasciae form the specific bands which hold the tendons near the bones when they pass over the flexion or extension angle of a joint (Retinacula tendinorum). 2. **The synovial bursae** (Bursa synovialis) serve as a gliding surface or as fluid-filled cushion which appears between the tendon and a bone prominence. The bursa has the same structure as the joint capsule, having an inner stratum (synoviale) and an external stratum (fibrosum). Their presence is not constant, depending on the age, the nutritional status and the usage. 3. **The synovial tendon sheaths** (Vagina synovialis tendinis) have a tube-like structure, being filled with the synovia. The vaginae tendinis envelop the tendons and pass over the hard skeletal areas, where the tendons are under a considerable tension. The synovial layer of the tendon sheath has the parietal layer, which is in contact with the stratum fibrosum that, through a pedicle (Mesotendineum), continues with the visceral layer, which envelops the tendon (Fig. 3. 3). ### 3.2. THE MUSCLES OF THE HEAD Depending on their topography and the nerve supply, the muscles of the head may be differentiated into three groups: * **The muscles of the face**, the mimetic muscles which receive the motor innervation from the facial nerve (VII); * **The chewing muscles**, muscles of mastication which receive the motor innervation from the trigeminal nerve (V); * **The muscles of the intermandibular space and the hyoid bone;** * **The other muscles that have a relation with the systems or sense organs:** * Muscles of the tongue, receiving the motor innervation from the hypoglossus nerve (XII); * Muscles of the pharynx, which receive the motor innervation from the glossopharyngeal nerve (IX) and vagus nerve (X); * Muscles of the larynx, innervated by the vagus nerve (X); * Muscles of the eye, that are innervated by the oculomotorius communis nerve (III), the trochlear nerve (IV) and the abducens nerve (VI); they are described as the motor adnexae of the eye-ball. #### The superficial and the deep fascia of the head The supeficial fascia covers the parotid gland, the masseter and the temporal muscles being adherent to the facial bones (in horses and ruminants). It encloses the cutaneous and some auricular muscles. The deep fascia extends over most of the mandible areas, some muscles are individually ensheathed, such as the buccinator and the canine muscles. Aborally, the deep fascia continues with the temporal fascia; in the dorsal region of the nose it merges with the superficial fascia. ### 3.2.1. THE MUSCLES OF THE FACE The facial musculature may be divided into: * **The muscles of the lips and cheeks;** * **The muscles of the nose;** * **The muscles of the eyelids;** * **The muscles of the external ear.** #### 3.2.1.1. The muscles of the lips and cheeks The muscles of the lips and cheeks rostrally and laterally delimitate the oral cavity (Figg. 3.4.-3.8.). 1. **The orbicularis muscle of the mouth** (M. orbicularis oris) represents the basic structure of the lips, being connected with the regional skin. Insertions: The muscular fibers are more or less circular, having no direct attachament to the incisive and mandible bones. Function: The muscle bounds the oral orifice, functioning as a sphincter of the mouth and exerts pressure on the labial glands. 2. **The incisive muscles** (M. incisivi): The incisive muscles are placed under the submucosa of the lips, being divided into: * **Pars maxillary incisive** (M. incisivus superior), * **Pars mandibularis** (M. incisivus inferior). Origin: rostral region of the mandible or incisive bone, along the alveolar border Insertion: the fibers mix with those of the m. orbicularis oris. Function: * When acting together (pars incisive and pars mandibularis), they press the lips against the arch of the incisors; * When acting alone: the maxillary incisive part raises the upper lip and the mandibular part pulls down the lower lip. 3. **The nasolabial levator muscle** (M. levator nasolabialis) (Figg. 3.4., 3.7.) The muscle appears flat and thin, its belly being divided into two branches by the canine muscle (except in pigs and carnivores). Origin: the muscle extends from the fascia of the frontal and the nasal regions. Insertion: the fibers insert on the upper lip together with other muscles and on the lateral wing of the nostril. Function: it elevates the upper lip and dilates the nostrils. 4. **The zygomatic muscle** (M. zygomaticus) (Figg. 3.4, 3.5, 3.9) The zygomatic muscle appears as a thin strip (in horses) or a powerful muscle (in ruminants), it follows the zygomatic arch horizontally, up to the corner of the lips. In carnivores, it is divided into several branches. Origin: the fibers originate below the facial crest, fascia masseterica and the zygomatic arch. Insertion: at the commissure of the lips, the fascicles are mixing with the same ones of the m. orbicularis oris. Function: the muscles retract the corners of the lips. 5. **The levator muscle of the upper lip** (M. levator labii superioris) The levator muscle forms a strong belly, which is partly covered by the m. levator nasolabialis (Figg. 3.4., 3.5., 3.6.). Origin: on the maxilla and the lacrimal bone, below the medial canthus of the eye (in horses) or the facial tubercle (in ruminants). Insertion: the fibers spread into the upper lip. In horses, the muscles continue with long tendons that fuse and form a common aponeurosis at the level of the nasal plane (between the two nostrils). Function: the muscles elevate and retract the upper lip. 6. **The caninus muscle** (M. caninus) (Figg. 3.6., 3.9.) The caninus muscle appears thin and quadrangular, being placed on the lateral side of the face. In horses and ruminants, the muscle passes between the two branches of the m. levator nasolabialis. Origin: on the lateral surface of the maxilla, close to the rostral extremity of the facial crest and the facial tubercle (in ruminants). Insertion: the fibers are distributed on the lateral wing of the nostrils (in horses, ruminants, pigs) or into the upper lip, in the case of dogs. Function: the muscle dilates the nostrils, it pulls back the upper lip or the snout in pigs. 7. **The depressor muscle of the upper lip** (M. depressor labii superioris) The muscle is present only in ruminants and pigs, being placed ventrally to the m. caninus (Fig. 3.9.). Origin: rostrally to the facial tubercle (in ruminants), or fossa canina (in pigs) Insertion: the tendon divides in many branches which form a network into the upper lip (in small ruminants) or the nasolabial plane (in large ruminants). In pigs, the tendons unite with the other muscles at the level of the snout. Function: the muscle acts as a depressor of the upper lip or snout (in pigs). 8. **The depressor muscle of the lower lip** (M. depressor labii inferioris) The muscle is absent in carnivores, in the rest of the species being placed ventrally to the buccinator muscle (Figg. 3.5., 3.6., 3.9.). Origin: along the alveolar border of the mandible (aborally), the maxillary tubercle and the coronoid process (in horses). Insertion: the fibers radiate into the lower lip. Function: it depresses and retracts the lower lip. 9. **The buccinator muscle** (M. buccinator) (Figg. 3.5., 3.6., 3,9.) The buccinator muscle forms the base of the cheek. In ruminants and horses, the muscle is formed by two parts: * **The superficial buccal part** (Pars buccalis) consists of longitudinal fibers which originate from the alveolar border of the maxilla and the mandible; * **The deep molar part** (Pars molaris) is more medio-caudally placed, it originates from the alveolar border of the cheek teeth and from the coronoid process of the mandible. Insertion: the fibers spread into the corner of the lips, Function: it presses the cheek to the teeth in order to return the food from the vestibule to the main oral cavity; it is involved in drinking liquids, as well. 10. **The mental muscle** (M. mentalis): The mental muscle forms the greater part of the fibromuscular pad of the prominence of the chin; it is rudimentary in carnivores. Origin: on the body of the mandible (the incisive part); Insertion: into the lower lip within the m. orbicularis oris fibers and the skin. Function: it retracts the lower lip. #### 3.2.1.2. The muscles of the nose The muscles of the nose surround the nostrils, being rudimentary or even absent in carnivores (Fig. 3.14) and pigs. Together with the m. levator nasolabialis and m. caninus, they are involved in respiration by enlarging the nostrils and increasing the air intake (Figg. 3.7., 3.8.). Depending on their position, in horses and ruminants, the muscles are divided into: 1. **The apical dilator of the nostrils** (M. dilatator naris apicalis) (Fig. 3.7.) It is an unpaired muscle, being inserted between the two alar cartilages, under the aponeurosis of the m. levator of the upper lip (M. levator labii superioris). Function: it dilates the nostrils. 2. **The lateral nasal muscle** (M. lateralis nasi) The muscle consists of (Fig. 3.8.): * **The superficial layer,** * **The deep layer.** In horses, the muscle is thin, surrounding the borders of the nasoincisive notch, it is divided into the dorsal, the ventral, the caudal and the rostral parts. The fibers spread into the skin of the diverticulum nasi. In ruminants, it is the most developed of the nasal muscles. Function: It acts as a lateral dilator of the nose. 3. **The medial dilator of the nose** (M. dilatator naris medialis) The muscle is present only in ruminants, being reduced. Origin: on the lateral nasal cartilage, Insertion: on the dorsal part of the medial angle of the nostril, Functions: They dilate the nostrils and the rostral part of the respiratory tract. #### 3.2.1.3. Muscles of the eyelids The muscles are placed around the orbit and act on the eyelids, raising, lowering or laterally and medially retracting them (Figg. 3.10., 3.11.). 1. **The orbicularis oculi** (M. orbcularis oculi) The muscle is the sphincter muscle of the eye, which divides into: * **A deep part** (Pars orbitalis) that is more developed and lies directly against the rim of the orbit, * **A superficial part** (Pars palpebralis) that is smaller, being placed within the eyelid layers. Function: it reduces the interpalpebral space and closes the eyelids. #### 3.2.1.4. The muscles of the external ear The ears in animals are very mobile, rotating 180 degrees. The auricular muscles are very developed and arise from the surrounding bones or from the scutiform cartilage (Cartilago scutiformis) and insert on the cartilage of the ear (Figg. 3.12., 3.13). The scutiform cartilage is a triangular plate which glides on the temporal muscle surface, being medio-rostrally placed to the auricular concha. The auricular muscles are gruped into: the scutular (mm. scutulares), the rostral (mm. auriculares rostrales), the ventral (mm. auriculares ventrales), the dorsal (mm. auriculares dorsales) and the caudal (mm. auriculares caudales) muscles of the ear. A. **The scutular muscles** (Mm. scutelares) form thin and fan-like fascicles that arise from the surrounding bones to the cartilago scutiformis: * m. fronto-scutularis originates from the frontal bone or continues the m. frontalis (in ruminants); * m. interscutularis inserts between the two scutiform cartilages; * m. parieto-scutularis (or cervicoscutularis) is more developed in carnivores and originates from the rostral surface of the parietal bone; * m. zygomatico-scutularis (M. zygomaticoscutularis) arises from the zygomatic arch to the scutiform cartilage, drawing the ear rostrally. * m. cervico-scutularis muscles (M. cervicoscutularis) passes from the nuchal crest to the scutiform cartilage, drawing the ear aborally. The superficial and deep fascicles pass from the scutiform cartilage to the ear, being represented by: * **The superficial scutulo-auricular muscles** (Mm. scutulo-auriculares superficiales): the dorsal, the middle and the ventral fascicles. They have a common insertion on the rostromedial side of the concha auriculae, raising the ear. * **The deep scutulo-auricular muscles** (Mm. scutulo-auriculares profundi), major and minor, are placed between the ventral surface of the scutiform cartilage and the base of the ear (Eminentia conchae); their fibers are crossed, rotating the ear. * **B. The rostral auricular muscles** (Mm. auriculares rostrales) are known as the aductor muscles of the ear and consist of: mm. scutulo-auriculares superficiales, m. scutulo-auricularis profundus, and m. zygomatico-auricularis. * m. zygomatico-auricularis arises from the zygomatic arch and inserts into the rostral part of the concha auriculae. * **C. The caudal auricular muscles** (Mm auriculares caudales) (Fig. 3.13.) * **The superficial cervico-auricular muscle** (M. cervicoauricularis superficialis), * **The middle cervico-auricular muscle** (M. cervicoauricularis medius), * **The deep cervico-auricular muscle** (M. cervicoauricularis profundus). Origin: they arise from the nuchal crest; Insertion: onto the convex caudal surface of the concha auriculae; Function: The muscles act as abductors and caudal retractors of the ear. * **D. The dorsal auricular muscles** (Mm. auriculares dorsales) are represented by the interscutular muscle (M. interscutularis), the parietoscutular muscle (M. parietoscutularis) and the parietoauricular muscle (Fig. 3.12.). * **The parietoauricular muscle** (M. parietoauricularis) arises from the parietal region and inserts into the scutiform cartilage and the dorsal surface of the concha auriculae. The muscles elevate the ear and draw it backward and forward. * **E. The ventral auricular muscles** (Mm. auriculares ventrales) consist of: * **The stylo-auricular muscle** (M. styloauricularis) is a very thin muscle band which slips over the meatus acusticus externus cartilagineus. It arises from the mandible (in carnivores) or pars tympanica ossis temporalis to the concha. It shortens the external ear duct (Fig. 3.14.). * **The parotido-auricular muscle** (M. parotidoauricularis) forms a thin and long muscles band which originates from the parotid fascia and inserts into the ventral margin of the ear. The muscle draws the ear ventrally and caudally (Fig. 3.11.). ### 3.2.2. The muscles of mastication The muscles involved in mastication are very developed and strong in all species, and they act on the temporo-mandibular joint (Figg. 3.11.-3.15.). 1. **The Masseter muscle** (M. masseter) is an extremely strong muscle, strenghtened by many interweaving tendons. It consists of two parts: * **The superficial part** (pars superficialis) that is covered by a strong fascia in horses. Origin: on the facial crest (tuber faciale in ruminants) and the zygomatic arch; Insertion: along the ventral margin of the fossa masseterica. * **The deep layer** (pars profunda) arises from the ventral border of the zygomatic arch and inserts into the fossa masseterica. Function: they bring the jaws together and tense them move the mandible towards the side of the contracting muscle when acting alone and raise the mandible and close the mouth. 2. **The Temporal muscle** (M. temporalis) occupies the entire fossa temporalis, from which it originates. Insertion: onto the coronoid process of the mandible; Function: it brings the jaws together, raises the mandible against the maxilla (the strongest levator of the mandible), closing the mouth. 3. **The Pterygoid muscles** (mm pterygoidei) are placed on the medial side of the mandible, being represented by (Fig. 3.15., 3.17.): * **The medial pterygoid muscle** (M. pterygoideus medialis) Origin: on the pterygoid process of the basisphenoid; Insertion: on the pterygoid fossa of the mandible; Function: when acting together, they raise the mandible, when acting alone, it produces the lateral displacement of the mandible. * **the lateral pterygoid muscle** (M. pterygoideus lateralis) is placed laterally to the medial pterygoid muscle, being the smallest muscle of mastication. Origin: on the pterygoid crest; Insertion: on the pterygoid fovea of the mandible, near the processus condylaris; Function: it draws the mandible rostrally and laterally. 4. **The digastric muscle** (M. digastricus) is formed from two fusiform bellies, the rostral and the caudal one, they are united by an intermediate tendon that is visible in horses and bovines and which passes through the insertions of the stylohyoideus muscle (Figg. 3.15., 3.16., 3.17.). In horses, the aboral belly branches to form a lateral portion of the occipito-mandibular muscle (M. occipitomandibularis) that inserts into the angle of the mandible (Fig. 3.16.). Origin: from the paracondylar process; Insertion: * the rostral part ends on the medial surface of the mandible; * the caudal part ends on the angle of the mandible or angular process; Function: it depresses the mandible downward and backward and thus it opens the mouth. ### 3.2.3. The muscles of the intermandibular space and hyoid muscles The muscles complete the intermandible space and are involved in sucking, drinking water and in deglutition (Figg. 3.15., 3.16., 3.17.). 1. **The stylohyoid muscle** (M.stylohyoideus) is a slender and spindle-like muscle. Origin: from the caudal part of the stylohyoid bone; Insertion: on the basihyoid bone; in horses and bovines, the insertion is perforated for the passage of the intermediate tendon of the digastric muscle; Function: the muscles retract the basyhyoid bone caudally and draw dorsally the base of the tongue, and the larynx dorso-caudally. 2. **The ceratohyoid muscle** (M. ceratohyoideus) is triangular in shape, being placed in the space between the ceratohyoid and thyrohyoid bones; it acts as a levator of the tyrohyoid bone and the larynx. 3. **The transverse hyoid muscle** (M. hyoideus transversus) is absent in pigs and carnivores, and, in the rest of the species it appears as a transversal band between the two ceratohyoid bones. It is unpaired and produces the elevation of the root of the tongue during deglutition. 4. **The mylohyoid muscle** (M. mylohyoideus) completes the intermandibular space, forming the floor of the oral cavity (Fig. 3.17.). It consists of: * the rostral part (pars rostralis), which originates from the mental angle of the mandible and from a median fibrous raphe; * the aboral part (pars caudalis), which originates from the caudal part of the mylohyoidian line; Insertion: both parts end on the lingual process of the basihyoid bone; Function: it raises the floor of the mouth and draws rostrally the tongue and the hyoid bone. 5. **The geniohyoid muscle** (M. geniohyoideus) is a long and spindle-like muscle which is placed above of mylohyoidian muscle, completing the intermandibular space (Fig. 3.15.). Origin: along the mental angle; Insertion: on the lingual process of the basihyoid bone; Function: it draws the hyoid bone and the tongue rostrally. 6. **The occipito-hyoid muscle** (M. occipitohyoideus) is a short and flat muscle that is found between the paracondylar process and the caudal end of the stylohyoid bone (Fig. 3.17.). Origin: from the paracondylar process of the occipital bone; Insertion: on the dorso-aboral extremity of the stylohyoid bone; Function: it dorso-caudally carries the stylohyoid bone. 7. **The sternohyoideus and the sternothyroideus muscles** are deeply placed, following the ventral face of the trachea. They both originate from the manubrium sterni. The muscles form a common belly up to the middle of the neck, then they separate into the sternothyroideus which inserts into the cartilago thyroidea and the m. sternohyoid, which inserts into the basihyoideum. Function: they retract and depress the hyoid bone, the base of the tongue and the larynx. 8. **The omohyoideus muscle** is a thin and ribbon-like muscle that obliquely passes from the scapula to the hyoid. Origin: from the transverse processes of the 3rd to the 5th cervical vertebrae and the subscapular fascia; Insertion: on the basihyoid bone, together with the m. sternohyoideus; Function: it retracts the hyoid bone and the root of the tongue and separates the jugular groove into the superficial and deep spaces (having clinical importance for intravenous injections). ### 3.3. MUSCLES OF THE CERVICAL, THORACIC AND LUMBAR VERTEBRAL COLUMN The muscles of the neck and trunk are called the muscles of the axial zone. Regarding their position and role, they are dorsally and ventrally placed to the transverse processes of the vertebrae producing the extension or flexion and the lateral movement of the spinal column. Depending on their topography, they are described as the muscles of the neck, trunk, abdomen, etc. The muscles that connect the legs to the axial skeleton are **synsarcotic**, both in the case of the thoracic and pelvic limbs. The **cutaneous muscle**, placed between the two blades of the superficial fascia of the body, is more or less developed in different regions, and the same in animals, having a role in tensing the fascia and inducing the skin tremor when the area is touched (Fig. 3.18.). ### 3.3.1. THE DORSAL MUSCLES OF THE NECK AND TRUNK The dorsal muscles of the neck and trunk are paired (except for the right muscles of the head) and are placed dorso-laterally between the transverse processes and the articular or spinous processes (or supraspinous and nuchal ligaments) of the vertebrae, having a role in the extension of the spine and the head (when the homologue muscles act together) and lateral movements (when the muscle acts alone). From the outer to the inner side, the dorsal muscles are arranged into four layers that are separated by the blades of the deep regional fascia. The muscles are placed into 4 layers. #### 3.3.1.1. The first layer (1) of the cervical and trunk muscles M. Trapezius, pars cervicalis and thoracis, they have a fan-shaped appearance because of their fusion, being extended from the head or neck to the caudal border of the scapula (Figg. 3.19.-3.21.). Origin: both parts are inserted in the funiculus nuchae and supraspinate ligament: * in horses it reaches the 2nd, * in ruminants from the 1st or 2nd, * in carnivores from the 3rd to the 10th-12th thoracic vertebrae. In pigs, it originates from the occipital crest, being more developped; Insertion: through a strong aponeurosis on the spine of the scapula; Function: it fixates the thoracic limb to the trunk (synsarosis) and in the abduction of the scapula, pulls the scapula forward (pars cervicis) and downward (pars thoracis. #### 3.3.1.2. The second (II) layer of the cervical and trunk muscles 1. **The rhomboideus muscle**, pars capitis (in pigs and carnivores) pars cervicis and pars thoracis (in the rest of the domestic mammals), is placed under the trapezius muscle, on the dorsal area of the neck and withers (Figg. 3.22.- 3.25.). Origin: from the funiculus nuchae, lig supraspinale, the processus spinossus in the extension of the 2nd (or 3rd) cervical vertebrae to the 4th (carnivores) or the 8th thoracic vertebrae (in the rest of the animals). In carnivores, the rhomboideus capitis passes over the splenius muscle. In horses, the cervical and thoracic parts fuse, appearing not distinct. In pigs, the thoracic part is well developed. Insertion: on the cranial (cervical) angle of the scapula (pars capitis and cervicis) and along the medial surface of the cartilago scapulae (pars thoracis). Function: It fixes the foreleg on the trunk and pulls the scapula forward. 2. **The omotransversarius muscle** is a thin and ribbon-like muscle that ventro-laterally lies to the m.trapezius cervicis, partly being covered by the brachiocephalic muscle (Figg. 3.21., 3.24.). Origin: on the transverse processes of the 2nd-4th cervical vertebrae and the wing of the atlas in dogs and ruminants; Insertion: through the deep fascia into the spine of the scapula; Function: it fixates the shoulder and the frontleg to the trunk and lifts it forward. 3. **The latissimus dorsi muscle** is one of the largest muscles in the body. It has a triangular aspect and transverses the lateral side of the trunk from the lumbar region to the foreleg. In ruminants and carnivores, it is thinner than in the case of horses and pigs. Origin: trough the aponeurosis from thoracolumbar fascia, lig. supraspinale of the lumbar and thoracic regions; The muscle fibers are cranio-ventrally oriented towards the humerus. Insertion: on the tuber teres major of the humerus together with the teres major muscle or on the tuber teres minor in carnivores; Function: it fixates the foreleg on the trunk, it flexes the shoulder joint, it may be active during forced breathing in. 4. **The ventral serrate muscle** is a large fan-like muscle that is divided into: * a. **the cervical part**, which is more developed in pigs. Origin: through the fascicles from the 3rd to the 6th cervical vertebrae (in horses, ruminants and carnivores) (Figg. 2.22., 2.25.) or on all transverse processes in pigs. Insertion: on the facies serrata cranialis of the medial face of the