Musculature

The muscular system is composed of three different types of muscles, which include skeletal muscles, smooth muscles, and cardiac muscles. Most of the muscles within the body are skeletal muscles, which usually attach to bones and function to generate voluntary movements of the limbs, trunk, and head. Furthermore, skeletal muscle has a striped appearance and is therefore termed striated muscle. Smooth muscle is unstriated and is under involuntary control. Moreover, it is found in sections of the body with autonomic function, which include the walls of organs in the digestive and urogenital systems as well as most blood vessels. Cardiac muscle is striated muscle that is not controlled voluntarily and it is solely found in the heart. The sole function of vertebrate muscles is contraction and movement of the specific bone or organ occurs when the muscle contracts and relaxes. Muscles are often attached to two bones in which the attachment site that is less mobile is termed the origin and the more mobile attachment site is termed the insertion. Muscles are also often grouped based on their function. The several different muscle functions include flexors, extensors, adductors, abductors, sphincters, cutaneous muscles, agonists, antagonists, and synergists.  

The Muscles of the Thoracic Limb

The muscles identified during the chicken dissection were m. pectoralis major, m. pectoralis minor, m. biceps brachii, m. triceps brachii, and m. deltoideus. The muscles of the thoracic limb of the chicken work to elevate and pull the wing downward as well as, flexing, extending, and rotating the various bones associated with the thoracic limb. Variation in skeletal muscle is most apparent in the wing of the chicken. The largest muscle of the bird is the m. pectoralis major, which is the major contributor to the downward movement of the wing. The muscle originates at the sternum, furcula, and sternal ribs. It inserts at the ventral surface of the humerus. The muscle that controls the elevation of the wing in the upstroke is the m. pectoralis minor (Supracoracoideus) and it lies deep to m. pectoralis major. This muscle originates in the sternum, furcula, and sternal ribs. It inserts at the proximal dorsal surface of the humerus. Movement of this muscle is facilitated by a tendon, which connects to the m. pectoralis minor and passes along the medial surface of the coracoid bone. The tendon then travels through an opening formed between the coracoid, scapula, and humerus, and finally inserts into the dorsal surface of the humerus. The muscle that aids the chicken in flexing its wing is the m. biceps brachii. It runs ventral to the humerus and its two heads originate near the glenoid fossa and proximal end of the humerus. It then inserts at the proximal anterior surface of the radius. The m. triceps brachii flex the shoulder and extend the forearm. Its two heads originate from the scapula and proximal humerus. The point of insertion of the m. triceps brachii is located at the olecranon process of the proximal ulna. The m. deltoideus also flexes the shoulder; however, it also aids in outward rotation of the wing. It is a sheet-like muscle that covers the dorsal shoulder and it originates at the proximal clavicle. Its point of insertion is the deltoid tuberosity of the proximal humerus (a picture is not shown because we did not look at the chicken dorsally). 

 

 

Muscles of the Pelvic Limb

The muscles of pelvic limb that were identified during the chicken dissection enable flexion, extension, and adduction of the bones within the pelvic limb. Muscles that flex joints decrease the angle between two sectors and are located on the side of the limb in which the joint bends. Conversely, extensor muscles work to increase the angle between to segments. While flexor and extensor muscles aid in forward and backward motion, adductors aid in pulling the limb toward the medial plane. The muscles observed that aid in these movements include the m. iliotibialis, m. sartorius, m. semiteninosus, m. semimembranosus, m. gastrocnemius, m. tibialis anterior, m. adductor longus, and m. quadriceps femoris. The m. iliotibialis, also known as the gluteus maximus, is a sheet like muscle that covers most of the thigh and works to flex the hip and extend the knee and lower leg. The muscle originates in the pelvis and inserts at the proximal tibia and patellar ligament. The m. sartorius also aids in flexion of the hip and extension of the knee; however, it originates at the lumbo-sacral area and inserts at the patellar ligament. Both the m. semitendinosus and m. quadriceps femoris aid in the extension of the thigh; therefore, they work to bring the thigh backwards. The m. semitendinosus originates at the ilium and ischium and inserts on the caudal femur. The m. semimembranosus extends the thigh and bends the knee. It originates at the pelvis on the lateral surface of the ischium and it inserts on the caudal surface of the tibiotarsus. Flexion of the knee is further aided by the m. gastrocnemius; however, this muscle also aids in extension of the foot. It has three heads and originates from the distal femur and proximal tibiotarsus. It then inserts at the tuber calcanei. Movement of the thigh is further supported by the m. tibialis anterior, which works to adduct and extend the thigh. This muscle originates at the ventral border of the ilium and pubis and inserts at the distal femur. The m. tibialis anterior assists in flexion of the tarsometatarsus forward. Its point of origin is at the anterior and distal femur and the proximal tibiotarsus. Its point of insertion is at the anterior surface of the tarsometatarsus. 

 

Abdominal Muscles  

The abdominal wall is mainly composed of muscles, which support the organs of the digestive and reproductive system. Abdominal muscles also aid in flexion of the vertebral column as well as defecation, micturition, and parturition. These muscles are arranged in layers and they are classified in regards to their location being either superficial or deep. External muscles are the most superficial, internal muscles are located right below external muscles, and transverse muscles are the deepest abdominal muscles. The fibers of abdominal muscles run in different directions; however, most abdominal muscles have aponeurtoic insertions that convene at the linea alba, a midventral line. The most superficial muscle of the abdomen is the m. obliquus externus abdominus and its fibers run in the ventral and caudal direction. Its point of origin is the last few rib and the thoracolumbar fascia. The muscle inserts at the linea alba by an aponeurosis, a broad flat tendon, located on the opposite side of the linea alba. The muscle that lies right below the m. obliquus externus abdominus is the m. obliquus internus abdominus. Although this muscle has the same points of insertion as the m. obliquus externus abdominus, it originates at the tuber coxae and its fibers run in the ventral and cranial direction. This muscle eventually gives rise to the cremaster m., which aids in pulling the testicles toward the abdominal cavity. The deepest abdominal muscle is the m. transverse abdominis, which originates from the deepest layer of thoracolumbar fascia. Similarly, it inserts on the linea alba; however, its fibers run perpendicular to the body axis. The floor of the abdomen is formed by the m. rectus abdominis, which is divided into tendinous intersections and its fibers run in the caudal direction. It originates from the cartilages of the ribs and the sternum and its point of insertion is at the pubis using a prepubic tendon. 

 

Muscles of the Head

The muscles of the head function in order to aid in mastication, facial expression, eye movement, deglutition, phonation, and tongue movement. The muscles of mastication attach to the mandible and move the jaw allowing the animal to chew. Facial muscles aid in movement of the skin and they also assist movement of hair and facial appendages.  They are often thin cutaneous muscles innervated by the facial nerve. The muscles of mastication include the m. temporalis, m. masseter, pterygoid muscles, and the m. diastricus. The m. temporalis brings the upper and lower jaw closer together. It originates from the sagittal crest and the expansive temporal fossa of the lateral cranium. Its point of insertion is the coronoid process of the mandible. The function of the m. masseter is to elevate the mandible and move it laterally. Its point of origin is the maxillary region of the face and zygomatic arch and its point of insertion is the caudal mandible. The pterygoid muscles are located medial to the mandible and originate at the pterygoid and palatine bones. There point of insertion is on the mandible. They aid in closing the jaw and side-to-side grinding movements. The opening of the jaw is facilitated by the m. digastricus, which originated at the region caudal to the temporomandibular joint and inserts on the angle of the mandible. The muscles of facial expression include the auricular muscles, m. orbicularis oculi, m. orbicularis oris, and m. buccinator. The auricular muscles are located rostral and caudal to the ear and aid in ear movement. The m. orbicularis oculi surrounds the opening of the eyelid, the palpebral fissure, and its contraction causes the eyelid to close. The m. orbicularis oris is another sphincter-like muscle and it surrounds the lips. Contraction of this muscle purses the lips. The final facial muscle is the m. bussinator, which forms the wall of the cheek. Other muscles of the head include the extraocular muscles, m. genioglossus, m. hyoglossus, and m. mylohyoideus. Extraocular muscles are striated muscles that lie within the orbit of the eye. These muscles aid in moving the globe of the eye. The m. genioglossus, m. hyoglossus, and m. mylohyoideus are all extrinsic muscles of the tongue, which means that they arise from the outside of the tongue. The m. genioglossus originates from the rostral part of the mandible and inserts in the base of the tongue. Its function is to draw the tongue outward. Conversely, the m. hyoglossus functions to pull the tongue backwards. It originates from the hyoid apparatus and inserts in the base of the tongue. The m. mylohyoideus contracts to lift the floor of the mouth and it is located transversely between the rami of the mandible.

 

Reproductive Tract Muscles

During the chicken dissection our chicken was a female, which is why we do not have photographs of reproductive tract muscles; however, the male reproductive tract muscle include the m. urethralis, the bulbospongiosus muscle, the ischiocavernosus muscles, the m. cremaster, and the rector penis muscles. All of the male genitalia muscles are striated muscles except for the retractor penis muscles, which are smooth muscles. The m. urethralis forms a voluntary sphincter around the urinary bladder. Its contraction also helps push semen through the urethra. The bulbospongiosus muscle is a continuation of the m. urethralis and it surrounds the urethra and the bulk of the penis. Furthermore, its contraction continues the process of emptying the urethra. The ishiovaernosus muscles cover the superficial region of the crura of the penis. They are paired muscles located on lateral sides of the body of the penis and their contraction pulls the penis dorsocraniad, which aids in erection. The retractor penis muscles are another group of paired muscles. These muscles originate from the first few caudal vertebrae and then continue ventrally down the anal canal towards the midline of the penis. The m. cremaster travels through the inguinal canal with the spermatic cord and then attaches to the outer layer of the testicles. This muscle aids in pulling the testicles towards the inguinal canal.