CLASS MAMMALIA 



427 



Enamel 



Pulp cavify 

 Dentine 



Cemen-hum 



significance is not definitely known. The opening from the pharynx 

 into the windpipe is the glottis. In connection with this opening is a 

 fleshy and cartilaginous structure called the epiglottis. At the junction 

 of the small and large intestines the latter is prolonged into a blind sac 

 called the caecum, which is enlarged in herbivorous mammals, where its 

 purpose seems to be to increase the capacity of the intestine. The 

 contracted tip of this caecum is known in higher primates including man, 

 as the vermiform appendix. Excepting in the egg-laying monotremes, 

 there is no cloaca, the anal opening being 

 at the surface of the body. 



The lungs are contained in coelomic 

 spaces called pleural cavities, which are the 

 lateral portions of the thoracic cavity, the 

 middle part of which is the pericardial 

 cavity. The thoracic cavity is separated 

 from the abdominal cavity by a thin, mus- 

 cular diaphragm, which is convex anteriorly 

 and concave posteriorly. The thoracic 

 cavity is expanded in breathing by raising 

 the ribs and flattening the diaphragm. 

 Thus air is drawn into the lungs. By the 

 relaxation of the muscles of the ribs and 

 diaphragm and the resulting contraction of 

 the thoracic cavity, air is forced out. At 

 the upper end of the windpipe, or trachea, 

 lies a larynx, or voice box. 



Mammals are warm-blooded animals 

 possessing a well-developed heat-regulatory 

 mechanism. The heart is four-chambered 

 and the systemic and pulmonary circula- 

 tions are entirely separate (Fig. 319). 

 There is a single aortic arch which turns to the left. A hepatic-portal 

 system is present but there is no renal-portal system. 



The brain of mammals (Fig. 320) shows a high development of the 

 cerebrum and cerebellum, the latter always being convoluted but the 

 former showing convolutions only in the higher forms. The size of 

 the cerebellum of mammals is equal to that of birds, while the cerebrum 

 far exceeds in its development that of any other vertebrate type. The 

 olfactory lobes are well-developed and each of the two optic lobes is 

 divided, making four altogether. 



The eye is without a pecten, which was present in birds, and the 

 nictitating membrane is reduced in size, varying in its degree of develop- 

 ment among the different groups. A spiral coiling of the cochlea and 

 the possession of a chain of three bones in the middle ear, together with 



^^'-''^O^-B/ooc/ vessel 



Fig. 318. — Canine tooth of a mam- 

 mal, diagrammatic. 



