2 THE ANATOMY OF THE HORSE. 



intercostal nerves, appear near the middle line, and are directed 

 transversely outwards. 



Directions. — The surface of the superficial pectoral muscles should 

 now be carefully cleaned by the removal of the subcutaneous fascia ; 

 and this operation should be conducted by beginning at the anterior or 

 posterior border of the muscle and working parallel to the direction of 

 the muscular fibres. When this has been effected a line will be seen 

 on the surface of the muscle, and by dissecting carefully down on this 

 line the student will be able to separate the anterior from the posterior 

 part of the muscle. Search is to be made, in the groove already men- 

 tioned, for the cephalic vein, and the fat is to be carefully removed from 

 the vein and its accompanying arterial branch. 



Superficial Pectoral Muscle (Pectoralis transversus of Percivall). — This 

 muscle is divided, though not very distinctly, into two portions, which 

 may be distinguished as the anterior superficial pectoral and the pos- 

 terior superficial pectoral. 



The Anterior Superficial Pectoral (Plate 1) arises from the first 

 two or three inches of the inferior border of the sternum, its posterior 

 fibres overlapping the anterior part of the next muscle. It is inserted 

 into the ridge which descends from the external tubercle of the humerus. 



The Posterior Superficial Pectoral (Plate 1) arises from the inferior 

 border of the sternum from within an inch of its anterior end as far 

 back as a point behind the 6th costal cartilage, and from a fibrous cord 

 which joins the muscle along the middle line to its fellow of the opposite 

 side. It is inserted into the superficial fascia which descends on the 

 inner face of the fore-arm ; and a few of its anterior fibres, forming a 

 band about one inch in breadth, are inserted into the humerus along 

 with the preceding muscle. At the elbow-joint the muscle covers the 

 posterior radial vessels and the median nerve, but these are not to be 

 exposed at present. 



Action. — The superficial pectoral muscle is an adductor of the limb at 

 the shoulder, and the posterior division of the muscle is also a tensor of 

 the fascia of the fore-arm. 



Directions. — Both divisions of the muscle are now to be cut across 

 near their origin, and dissected carefully from the subjacent deep 

 pectoral ; and, while this is being done, search is to be made for their 

 nerves, which come from the brachial plexus by passing between the 

 two divisions of the deep pectoral muscle. In reflecting the muscle 

 the dissector will cut many small branches of the external or internal 

 thoracic vessels. The reflected muscles are now to be fastened out- 

 wards with chain and hooks, and the dissection of the deep pectoral is 

 to be undertaken after the cephalic vein has been examined. 



The Cephalic Vein (Plate 1). This is the upward continuation of 

 one of the divisions of the internal subcutaneous vein of the fore-arm. 



