MAMMALIA 



281 



the contraction of its muscles. The cavity of the thorax is 

 thereby enlarged, and by the relaxation of the muscles it falls 

 into its condition of rest again. Thus a certain amount of 

 air is introduced during inspiration, and the same amount is 

 expelled during expiration. The air remaining in the lungs 

 after the expiration is called residual air. It is obvious that 

 the quantity of air introduced and expelled depends upon 

 circumstances of rest and running. 



Coelom. -The coelomic mesoderm is resolved into dorsal 

 myotomes and splanchnic mesoderm. The former yields the 



Neural spine 



Art. proc. 



Trans, proc. 



Centrum 



Aorta 



Neural spine 

 Articular process 



Sternum' 



FIG. 135. Transverse sections through thorax and abdomen diagrammatic to 

 illustrate the relationship of the membranes of the body cavities. 



muscles of the body and limbs and the dermis. The latter 

 forms the body cavity. The body cavity of the rabbit is 

 resolved into a peritoneal cavity and into pericardial and 

 pleural cavities. 



The peritoneal cavity is formed of a parietal layer lining 

 the inner side of the body wall, and it is reflected as a double 

 median fold dorsally to form the mesentery and the invest- 

 ment of the viscera. In the rabbit and other mammals the 

 folding of the intestine is accompanied by a great deal of 

 folding of the mesentery, but throughout it all it maintains 

 its continuity. Anteriorly in relation to the stomach, liver, 

 and diaphragm, the folds of the mesentery are called ligaments 

 and omenta. The inner aspect of the wall of the peritoneum 

 is smooth and it is kept moist. The movements of the 



