236 THE MUSCLES. 



1. Great Serratus. (Fig. 105, 15.) 



Synonyms. Costosubscapularis Girard. Posterior portion of the serratus magnus of 

 Bourgelat. (A portion of Percivall's serratus magnus.) 



Form Situation. A very wide muscle, disposed like a fan, split up 

 into digitations at its inferior border, applied against the thoracic walls, and 

 partly concealed by the shoulder. 



Structure. It is composed of divergent fleshy fibres, all of which con- 

 verge towards the superior extremity of the scapula, and are covered by 

 a very strong aponeurosis that gradually diminishes from above to below, 

 and only adheres to the muscle in its inferior part. 



Attachments. 1, To the external face of the eight sternal ribs ; 2, To 

 the anterior triangular surface of the internal face of the scapula, behind the 

 angularis, with which it is confounded; 3, To the whole extent of the 

 posterior triangular surface of that bone. 



On reaching the scapula, the aponeurosis separates from the fleshy 

 fibres, and is inserted alone into the fibrous plane which covers the muscular 

 fasciculi of the subscapularis. 



Relations. The great serratus responds : outwardly, and through the 

 medium of an abundant supply of cellular tissue, which facilitates the play 

 of the limb against the lateral wall of the thorax, to the subscapularis, 

 supraspinatus, the adductor of the arm, great dorsal (latissimus dorsi), and 

 the mass of olecranian muscles ; inwardly, to the first seven external inter- 

 costals, to the sides of the sternum, and the anterior small serratus. Its four 

 posterior digitations cross the first five of the great oblique muscle of the 

 abdomen, and are covered by a prolongation of the abdominal tunic. 



Action. With that of the opposite side, this muscle constitutes a vast 

 brace or girth on which the thorax rests when the animal is supported on 

 its limbs ; it therefore acts, in relation to the trunk, like a suspensory 

 ligament. When it contracts, its fixed point being the thoracic walls, it 

 pulls the superior extremity of the scapula downward and backward, and 

 causes this portion of the limb to perform a swinging movement which 

 carries the inferior angle upwards and forwards. If the limb is the fixed 

 point, then it raises the thorax between the two anterior members, and 

 assists in the respiratory movements by elevating the ribs. 



2. Transverse Muscle of the Ribs. (Fig. 105, 16.) 

 Synonyms. Costo-sternalis Girard. (Later alls sterni Percivall.) 



Form Structure Direction Situation. This is a flattened band, 

 aponeurotic at its extremities, oblique from upwards and forwards, and 

 situated under the preceding muscle, at the inner aspect of the deep pectoral. 



Attachments. Its posterior extremity is fixed to the sternum and the 

 fourth sternal cartilage ; the anterior to the external face of the first rib. 



Relations. Inwardly, with the second and third ribs, into which are 

 often inserted some of its fasciculi, and with the three first intercostal 

 muscles. Outwardly, with the two portions of the deep pectoral muscle. 



Action. This is an auxiliary to the expiratory muscles. (Leyh says its 

 action is to raise the ribs and their cartilages, and thus to enlarge the 

 anterior portion of the thorax during inspiration.) 



