:: THE ANATOMY OF THE HORSE. 



costal nerves, appear near the middle line, and are dix-ected transversely 

 outwards. 



Directions. — The surface of the superficial pectoral muscles should 

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

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

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

 the muscular fibi-es. 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 tlie 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 f;\t is to be carefully i-emoved fi'om 

 the vein and its accompanying arterial branch. 



Superficial Pectoral Muscle (Pectoralis transversxis 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 external lip of the musculo-spiral gi'oove. 



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 along with the preceding 

 muscle into the external lip of the musculo-spiral groove. 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 x*eflecting the muscle, the dissector will 

 cut many small branches of the external or internal thoracic vessels. 

 The reflected muscles are now to be fastened outwards 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. 



