8 Practicum I. The Pectoral Muscles. 



a. Taking care not to cut the pectorals, divide the skin and skin- 

 muscles around the arm just proximad of the elbow. Do the same for the 

 right arm and then, if necessary, for the legs, at about the middle of the 

 thighs. Finally cut the skin just caudad of the ears. 



b. If preferred the skin may now be divided with the scissors along 

 both mesons (dorsal and ventral median lines) ; or it may be removed from 

 the right side by dissecting it up from either or both mesons. The skin 

 of the head should be left until that region is dissected. 



c. Examine the ental aspect of the skin and note the pale sheet of 

 skin-muscle, with close adhesions, especially caudad ; it narrows and 

 thickens cephalad and is connected with muscles attached to the arm. 



14. Place the cat on its back and tie the arms outstretched to the 

 loops at the side of the tray. Compare PI. III. Recognize the convex 

 shoulders and the axillas (arm-pits). 



The MESON (middle line) may not be perfectly straight on account of 

 the twisting of the body. At the meson may be recognized the EPI- 

 GASTRIUM (pit of the stomach), and the STERNUM (breast-bone) extending 

 thence to the root of the neck PI. II, /. 



15. The Pectoral Muscles. These form, as in man, a considerable 

 mass covering the ventral aspect of the thorax and extending thence to 

 the brachium. Read the description on PI. Ill, and note the differences 

 from the human. 



The cephalic margin of the pectoral mass overlaps the muscles on 

 the ventral aspect of the neck ; it extends nearly transversely to the con- 

 vexity of the shoulder. The cephalic margin is sometimes distinct, but 

 sometimes quite thin and so closely adherent to the cervical muscles as to 

 separate with some difficulty. The caudal margin is thinner and extends 

 obliquely from the axilla to the epigastrium. 



16. Transecting the Pectorals. Compare Plates III and IV. 



a. At a point on the cephalic margin about (not more than) one- 

 fourth of the distance from the meson to the convexity of the shoulder, 

 begin an incision which is to be carried caudad parallel with the meson 

 to the caudal margin ; it should be not more than 3 mm. (one-eighth 

 inch) deep ; the division of the ectal layers will expose ental layers not 

 quite parallel therewith. 



b. Divide the ental layers by a second incision of about the same 

 depth, so as to reach a muscle, the RECTUS, whose direction is parallel 

 with the meson instead of at an angle with it. 



c. Just ental of the pectorals are some nerves and blood-vessels 

 which are not to be cut at this time. 



17. The Axillary Parts. Lift the lateral portion of the pectoral 

 mass and partly tear, partly cut it toward the arm. In the interval be- 

 tween it and the arm and the thorax may be seen : 



a. Web-like CONNECTIVE TISSUE, composed of delicate fibers. 



b. In well-nourished animals, masses of firm white FAT. 



c. Several NERVES, white and solid. 



d. A smaller number of BLOOD-VESSELS, elastic, hollow, and some- 

 times containing blood. The main ARTERY usually lies a little cephalad 

 of the VEIN (PI. 4), is more elastic and probably more nearly empty. 



f. The axillary parts should be exposed with the tracer rather than 

 the scalpel. 



$ iS. The ( '/(ir/< It (collar-bone). Note its location on PI. I. Feel 

 for it among the transected muscles at the shoulder ; it is usually laterad 



