2 DISSECTION OF THE DOG 



Dissection of the Male. — If the subject be a male, make an incision along 

 the mid-ventral line from the middle of the neck to the prepuce. Then 

 make a transverse incision from the medial aspect of one elbow to a 

 similar point on the opposite hmb. Turn back four flaps of skin. 

 In doing so, an extensive but thin subcutaneous muscular sheet 

 — m. cutaneus — will be exposed in the neck, where its fibres are 

 transverse, and over the abdomen, where they are oblique. 



Define the pectoral muscles. 



In clearing the fascia from the pectoral muscles note small vessels and 

 nerves (about six), which appear close to the mid- ventral line and 

 proceed laterally over the surface of the muscles. These vessels are the 

 perforating branches of the internal mammary artery. In the xiphoid 

 region an artery and vein (branches of the cranial epigastric vessels) 

 appear from under the border of the deep pectoral muscle, and run 

 obUquely in caudo-lateral direction to supply the mammary region. 

 These are larger in the female. 



M. PECTOEALis suPBRFiciALis. — In the main the fibres of the superficial 

 pectoral muscle run transversely to the long axis of the body. The origin of 

 the muscle is from the first two segments of the sternum and from the septum 

 between it and its fellow muscle, and its insertion — hidden as yet by the 

 brachio-cephalic muscle — is to the line on the humerus running distalwards 

 from the tuberculum majus. 



Crossing the superficial pectoral muscle close to its insertion, and lying in 

 a narrow space bounded by this and the brachio-cephalic muscle, are two 

 vessels : namely, a communicating branch from the cephalic vein to the external 

 jugular vein, and the deltoid ramus of the thoraco-acromial artery. 



Dissection. — Reflect the superficial pectoral muscle by cutting across it 

 close to its origin, and notice in doing so that the muscle increases in 

 thickness towards its cranial border. Clean the surface of the deep 

 pectoral muscle and dissect out the various structures lying in a triangular 

 space at the root of the neck. In reflecting the superflcial muscle observe 

 its nerve of supply (from the brachial plexus) bending round the cranial 

 border of the deep pectoral muscle, accompanied by the pectoral ramus 

 of the thoraco-acromial artery and its satellite vein. 



M. PECTORALis PROFUNDUS. — The deep pectoral muscle is much more 

 extensive than the superficial member of the same group. Consequently, a 

 considerable extent of it is visible before the superficial ra^Jj^ is reflected. 

 Its fibres are disposed Avith a varying degree of obhquity. *rhe most caudal 

 fibres are the most oblique. The caudal border of the muscle is rendered some- 

 what indefinite from the presence of a narrow band sometimes isolated, and 

 always separable, from the rest. 



