2 TOPOGRAPHICAL ANATOMY OF 



known as the clavicular portion (pars clavicularis), on the assumption 

 that it corresponds to that part of the pectoral which arises from the 

 clavicle 1 in those animals in which this bone exists. In the horse, 

 where the clavicle has disappeared, the muscle has shifted its origin 

 to the nearest bone, namely, the sternum. The caudal or sternocostal 2 

 part (pars sternocostalis) of the muscle is more extensive and composed 

 of bundles of fibres converging somewhat as they proceed towards the 

 elbow joint. 



It is not altogether easy to separate the clavicular and sterno- 

 costal parts of the pectoral muscle ; nor is it of much profit to do so. 

 If the dissector desires to make the separation he should look care- 

 fully for a very indistinct groove at no great distance from the 

 cranial border of the muscle, and, in effecting the separation, he 

 should bear in mind that the sternocostal portion of the muscle 

 overlaps the clavicular. 



The origin of the superficial pectoral muscle is from the sternum 

 from its cranial end to a level with the sixth rib-cartilage. It will 

 be noticed that some of the fibres spring from a fibrous partition 

 between the muscles of the two sides of the body. Its insertion is 

 into the crest of the greater tubercle of the humerus and to the 

 fascia lining the inner aspect of the forearm. 



Dissection. — The superficial pectoral muscle is to be reflected by 

 making a longitudinal incision through it at a short distance from 

 its sternal attachment. Care should be taken to avoid injury to the 

 underlying deep muscle, as the two pectorals are generally intimately 

 associated with each other close to the sternum. As the superficial 

 muscle is being raised and turned outwards, its nerve of supply should 

 be noted, as it pierces the narrow interval between the two parts of 

 the deep muscle. 



The surface of the deep pectoral muscle must be cleaned and its 

 borders defined. In doing this, an examination should be made of a 

 triangular space bounded by the pectoral and brachiocephalic muscles 

 and a strong band of the cutaneous muscle attached to the extremity 

 of the sternum. In examining the contents of the space the dissector 

 of the limb should work along with those who are dissecting the head 

 and neck. The triangle contains a group of lymph glands, the cephalic 

 vein, and the transverse scapular artery. Deep down in the space is 

 the jugular vein. 



The definition of the caudal border of the deep pectoral must be 

 conducted with care, for it is overlapped by the cutaneous muscle, and 

 associated with it are the external thoracic vein and the caudal pectoral 

 nerve. 



1 Clavicula [L.j, dim. of claims, a nail, or of clavis, key ; probably from K-Aets 

 [Gr.], key or bolt, collar-bone. The clavicle of the horse has disappeared, and is 

 now represented by a fibrous line in the brachiocephalic muscle. In the dog it 

 is a rudimentary bone embedded in that muscle. 



2 Sternum [L.], crrepvov (sternon) [Gr.], breast or chest. Costa [L.], rib. 



