254 FASCIA AND MUSCLES OF THE HORSE 



border of the supraspinatus. It is loosely attached to the latter muscle, and 

 terminates in a pointed end which becomes more firmly attached near the cer- 

 vical angle of the scapula. 



Relations. — Superficially, the skin and fascia, the panniculus, superficial pec- 

 toral, trapezius, and mastoido-humeralis muscles, the cephalic vein, and the 

 inferior cervical artery; deeply, the posterior deep pectoral, biceps, supraspi- 

 natus, omo-hyoideus, and serratus magnus muscles, the brachial vessels, and the 

 branches of the brachial plexus of nerves. 



(b) Posterior deep pectoral (portio humeralis s. ascend ens; pectoralis 

 magnus). — This is much the largest of the pectoral group in the horse. It is 

 somewhat triangular or fan-shaped. 



Origin. — (1) The abdominal tunic; (2) the xiphoid cartilage and ventral 

 aspect of the sternum; (3) the cartilages of the fourth to the ninth ribs. 



Insertion. — (1) The internal tul^erosity of the humerus; (2) the external lip 

 of the bicipital groove; (3) the tendon of origin of the coraco-brachialis. 



Action. — To adduct and retract the limb; if the limb is advanced and fixed, 

 to draw the trunk forward. 



Structure. — This muscle is also almost entirely fleshy. Its posterior part is 

 wide and thin, but as the muscle is traced forward, it becomes narrower and much 

 thicker. It passes forward and slightly upward in a gentle curve to its insertion. 

 The humeral insertion is just below that of the inner division of the supraspi- 

 natus. Part of the fibers are inserted by means of a tendinous band which binds 

 down the tendon of the biceps and is attached to the external lip of the bicipital 

 groove, and a small part is attached to the tendon of origin of the coraco-brachialis. 



Relations. — Superficially, the skin, panniculus, and superficial pectoral; 

 deeply, the abdominal tunic, the external oblique, the rectus abdominis et thoracis, 

 the brachial vessels, and branches of the brachial plexus of nerves. The external 

 thoracic vein lies along the outer or upper border. 



Blood-supply. — Internal and external thoracic, inferior cervical, anterior 

 circumflex, and intercostal arteries. 



Nerve-supply. — Pectoral (or thoracic) nerves, from the brachial plexus. 



4. Serratus magnus (M. serratus ventralis). — This is a large, fan-shaped 

 muscle, situated on the lateral surface of the neck and thorax. It derives its 

 name from the serrated ventral edge of its thoracic portion. It consists of a 

 cervical and a thoracic portion. 



(a) Cervical part (M. serratus cervicis; levator scapula? hominis). 

 Origin. — The transverse processes of the last four or five cervical vertebrae. 

 Insertion. — The anterior triangular area on the costal surface of the scapula 



and the adjacent ])art of the cartilage. 



(b) Thoracic part (M. serratus thoracis; serratus anterior hominis). 

 Origin. — The external surfaces of the first eight or nine ribs. 



Insertion. — The posterior triangular area on the costal surface of the scapula 

 and the adjacent part of the cartilage. 



Action. — The two muscles form a sort of sling in which the trunk is sus- 

 pended. Contracting together, they raise the thorax; contracting singly, the 

 weight is shifted to the liinb on the side of the muscle acting. The two parts 

 can contract separately and are antagonistic in their action on the scapula. 

 The cervical part draws the base of the scapula toward the neck, while the thoracic 

 part has the opposite action; these effects concur in the backward and for- 

 ward swing of the limb respectively. With the limb fixed, the cervical part 

 extends (raises) the neck or inclines it laterally. The thoracic part may act as a 

 muscle of forced ins])iration. 



Structure. — In the domesticated animals there is no such clear division of 

 the muscle as is found in man and the apes. On account of the difference in 



