Organs of Respiration and Circulation. 587 



Fig-. 8. 



Represents the heart anl left lung in situ, the side of the chest being cut away. 



tion. Such not being needed in the skull or pelvis, the bones 

 forming these cavities are firmly and immovably united together. 

 The thorax is bounded above by the spine, on the sides by the ribs, 

 below by the breast-bone or sternum, and behind by the diaphragm ; 

 a muscular tendinous partition separating it from the abdomen. 



The form of the thorax is that of a truncated cone placed hori- 

 zontally, having its apex formed by the near approximation and 

 shortness of the first pair of ribs, and its base by the diaphragm. 

 The dimensions of the cavity are consequently increased from before 

 backwards ; while the hinder part, or base of the cone, is cut off 

 obliquely from above downwards and forwards. The first pair 

 of ribs are situated nearly perpendicular ; and more especially in 

 the ox, where they form a right angle with the spine. One of 

 these is represented in situ in fig 8. The ribs of the horse num- 

 ber eighteen on either side, but in the ox and sheep they are only 

 thirteen. They increase in length from the first to the eighth, and 

 likewise in their curve obliquely backwards from the spine, from 

 the first to the thirteenth ; but they gradually diminish in length 

 from the eighth to the last. Their interspaces are filled up by 

 muscular fibres, the intercostal muscles, which are active agents 

 in inspiration. The ribs therefore with the diaphragm form the 

 moveable boundaries of the chest; the spine and the sternum 

 being more or less the fixed points from which they act. 



As seen in the annexed figure, No. 9, the ribs are united to the 



