30 DISSECTION OF THE CAT 



5. The Diaphragm is the musculo-tendinous par- 

 tition separating the abdominal and thoracic cav- 

 ities. The muscular portion of the diaphragm at 

 its circumference arises ventrally from the ensi- 

 form cartilage, laterally from the last five ribs and 

 the aponeuroses of the muscles of the back, and 

 dorsally from the bodies of the lumbar vertebrae 

 by two thick slips or critra. From all these 

 points the fibres converge, to end in the central 

 tendon. The diaphragm is pierced by the oesoph- 

 agus, aorta, and inferior vena cava. 



MUSCLES OF THE THORAX. 



i. The Pectoralis Muscle is a very large triangular 

 muscle, having its origin on the sternum and the 

 cartilages of the ribs and its insertion on the hume- 

 rus. It is made up of the following five por- 

 tions : 



a. A long, narrow, superficial division which 

 arises beneath the manubrium and the cartilages 

 of the first two ribs. Part of this is inserted in 

 the fascia of the fore-leg, and part of it joins one 

 of the shoulder muscles, the cep halo-humeral. 

 (Plates III. and IV.) 



b. The largest division arises from the manu- 

 brium and anterior third of the sternum, and is in- 

 serted on the outer side of the deltoid ridge of 

 the humerus. (Plate IV.) 



c. Another division arises from the sternum, be- 

 tween the cartilages of the second and sixth ribs. 



