MAMMALS 307 



partition, the diaphragm, into two complete cavities, the 

 thorax and abdomen. In this respect the mammalia differ 

 from all other living vertebrates. Usually the diaphragm 

 lies transversely to the long axis of the body. In the rabbit, 

 then, it lies vertically and in man it hes horizontally. 



The thorax, which is the anterior cavity, is bounded by 

 the ribs and contains the lungs and heart. The abdomen 

 is bounded by soft muscular walls and contains the liver, 

 stomach, intestines, reproductive and excretory organs. 



Respiration of the rabbit. — On the floor of the pharynx, 

 posterior to the tongue, is an opening, the glottis, that leads 

 into the windpipe. The glottis is closed by a Udlike organ, 

 the epiglottis, which prevents the food from falling into the 

 windpipe. The larynx, or voice box, is situated at thef 

 upper end of the windpipe which divides into two branches 

 before reaching the lungs. The lungs are lobed and filled 

 with air sacs bounded by thin membranous tissues per- 

 meated with capillaries. 



In taking in air the diaphragm is pulled backward by 

 the contraction of its muscles and the walls of the thorax 

 are expanded. These movements enlarge the thoracic 

 cavity, which allows the air to rush into the lungs through 

 the windpipe. An expiration is accomplished by the re- 

 verse movements. 



Circulation. — The circulation of the rabbit is very 

 similar to that of the sparrow. The heart is divided into 

 two complete halves with the right auricle and right ven- 

 tricle on one side and the left auricle and left ventricle on 

 the other. 



The blood flows from the different parts of the body into 

 the right auricle and thence into the right ventricle. From 

 the latter it is sent through the pulmonary arteries to the 



