The limb muscles of the mammal 



A study of the musculature of the mammal is generally 

 considered an important part of a student's training in com- 

 parative anatomy because of its dissection interest, the ter- 

 minology involved, and its value as an introduction to 

 human myology. Any comparative discussion of myology 

 must be based upon a knowledge of the musculature of at 

 least one animal. For this purpose, the cat will be described. 



The musculature of the limbs can be divided into ex- 

 trinsic and intrinsic divisions. Extrinsic refers to those mus- 

 cles anchoring the limb to the body; intrinsic identifies the 

 muscles within the limb. These are arbitrary divisions which 

 are hard to differentiate in the actual specimen. 



The muscles of the pectoral appendage After drawing the 

 skin off the shoulder region and exposing the upper part of 

 the forelimb, the dorsal extrinsic muscles can be observed. 

 Arising from the dorsal midline (the tips of the neural spines 

 of the vertebrae — Figure 14-5) are a series of sheet-like 

 muscles which taper toward and insert over the shoulder 

 region. The first of these, the levator scapulae, arises in the 

 occipital region and extends posterolaterally, below a part 

 of the second muscle, to insert on the spine of the scapula. 

 The next posterior muscle, the trapezius, is divided into 

 three parts: the pars clavotrapezius, the pars spinotrapezius 

 anterior, and the pars spinotrapezius posterior. The last 

 two parts insert along the spine of the scapula, and the first 

 inserts on the clavicle. The broad aponeurosis, or tendon, of 

 origin of the pars spinotrapezius anterior is transparent, and 

 through it can be seen a part of the supracoracoideus mus- 

 cle, whose fibers run forward and laterally. The trapezius 

 extends beyond the clavicle, to the forearm, as the pars 

 clavobrachialis. This could be considered an intrinsic mus- 

 cle. The latissimus dorsi, the most posterior muscle, arises 

 over a wide arc of the body wall. The belly tapers anteriorly 

 and inserts on the underside of the proximal end of the 

 humerus. The latissimus is partly overlaid with a sheet of 

 muscle which tapers to its origin in the axilla of the fore- 

 limb. This sheet is a part of the cutaneous maximum, which 

 twitches the skin (a cutaneous muscle). An intrinsic part of 

 the latissimus, the extensor antibrachialis, is described below. 



The several parts of the trapezius can now be cut, across 

 the middle of their bellies, and the cut ends reflexed. The 

 rhomboideus muscle is now exposed. It arises from the body 

 wall, along the neural spines; its fibers extend posteriorly 

 and then laterally, as numerous separate bundles, to insert 

 on the posteroventral margin and angle of the scapula. This 

 same area serves for the insertion of a second muscle, the 

 serratus anterior and posterior. The two parts of the ser- 

 ratus are not distinct; the fibers of the anterior are poste- 

 riorly directed from their slips of origin, off the rib bsisket, 

 to their insertion, while those of the posterior are dorsally 

 directed. 



Viewed ventrally (Figure 14-6), there are also several 



sheet-like muscles. The most anterior band, which extends 

 outward to insert on the forearm, beyond the elbow joint, is 

 the pars pectoantibrachialis anterior of the pectoralis major 

 muscle. Behind this and with a similar insertion may be a 

 small band from the pars humeralis; this second band is the 

 pectoantibrachialis posterior. The pars humeralis may be 

 subdivided, and behind it is the pars xiphihumeralis. If the 

 pectoantibrachialis and anterior portion of the pars humer- 

 alis are separated, deeper bands of the pectoralis are ex- 

 posed; these insert nearer the proximal end of the humerus 

 (and over the surface of the biceps muscle). The most medial 

 of these deep bands corresponds in position and function to 

 the pectoralis minor muscle of the human. In mammals 

 generally, the pectoralis is highly variable in its subdivisions. 

 Since homologizing the subdivisions is not possible, the 

 names used above are arbitrary. 



Returning to the dorsal aspect, the "intrinsic" muscles of 

 the upper arm can now be examined. With the trapezius 

 muscles reflexed, the muscles of the scapula are largely ex- 

 posed. The deltoideus has two parts: the first, the pars 

 acromialis, arising from the anterior end of the spine of the 

 scapula (the acromial process), and the second, the pars 

 spinalis, further back. These areas of origin are separated 

 by the area of insertion of the levator scapulae. Both parts 

 of the deltoid insert together on the anterolateral ridge 

 (deltoid crest) of the humerus. 



The scapula is covered by the supracoracoideus; above 

 the spine it is covered by the pars supraspinatus and below 

 the spine by the pars infraspinatus. Below the infraspinatus, 

 near its insertion on the head of the humerus, is the teres 

 minor. This is observed by cutting through the middle of 

 the deltoid bellies and reflexing the cut ends. Along the 

 ventral margin of the scapula is the large teres major mus- 

 cle paralleling the infraspinatus; its insertion end passes to 

 the underside of the upper-arm muscles. 



By clearing out the fascia, connective tissue, and separat- 

 ing the several deep bellies of the pectoralis, the medial as- 

 pect of the scapula can be observed. The fan-shaped ser- 

 ratus anterior and posterior arises off the body wall by a 

 number of slips; the belly tapers to its sertion at the poste- 

 rior tip of the scapula. Along the ventral margin of the 

 scapula is the teres major which tapers to a band and in- 

 serts on the inner aspect of the anterior crest of the humerus. 

 This insertion passes below, next to the humerus, the origin 

 tendon of the biceps muscle. Above the teres major is the 

 large subscapularis covering the inner surface of the scapula. 

 It tapers to an insertion on the adjacent head of the hu- 

 merus. Anterior to and above the insertion of the subscapularis 

 is the coracobrachialis muscle. This arises by a tendon 

 from the coracoid process of the scapula and expands to its 

 fleshy insertion on the head of the humerus next to the in- 

 sertion of the subscapularis. 



The muscles of the dorsal aspect of the upper arm include 

 the clavobrachialis, already described, which inserts with 

 the brachialis. The brachialis lies anterior to and below the 



424 • THE MUSCULAR SYSTEM 



