CHARACTERS OF MAMMALIA. 547 



As regards the digestive system of the Mammalia^ salivary 

 glands are present in all except the true Cetacea. The alimen- 

 tary canal has in most cases essentially the same structure as in 

 man ; and the same accessory glands are present namely, the 

 liver and pancreas. Some very remarkable modifications occur 

 in the structure of the stomach and in the termination of the 

 intestine ; but these will be noticed in speaking of the orders 

 in which they occur. The cavity of the abdomen is always 

 separated from that of the thorax by a complete muscular 

 partition the diaphragm as is the case in no other Vertebrate 

 animals. The abdomen contains the greater portion of the 

 alimentary canal, the liver, spleen, pancreas, kidneys, and other 

 organs. The thorax mainly holds the heart and lungs. 



The heart is contained in a serous bag, the pericardium, and 

 consists (as in Birds) of two auricles and two ventricles. The 

 effete and deoxygenated blood is returned from the tissues by 

 the veins, and is conducted by the two venae cavae to the right 

 side of the heart into the right auricle. From the right auricle 

 it passes into the right ventricle, whence it is propelled through 

 the pulmonary artery to the lungs. Having been submitted to 

 the action of the air, the blood, now arterialised, is carried by 

 the pulmonary veins to the left auricle, and thence into the left 

 ventricle. From the left ventricle the aerated blood is driven 

 through the aorta and systemic vessels to all parts of the 

 body. In Mammals, therefore, as in Birds, the pulmonary 

 and systemic circulations are altogether distinct and separate 

 from one another. The two sides of the heart except in the 

 foetus and as an abnormality in adults have no communica- 

 tion with one another except by means of the capillaries. 



The red blood-corpuscles are never nucleated, and in all 

 except the Camelidce (in which they are oval) they are circular 

 and discoid. 



The lungs of Mammals differ from those of Birds in being 

 freely suspended in the thoracic cavity, the greater part of 

 which they fill, and in being enclosed freely in a serous sac 

 (pleura] which envelopes each lung. The lungs are minutely 

 cellular throughout, and the bronchi never open on the sur- 

 face of the lung into a series of air-receptacles communicating 

 with one another, and placed in different parts of the body, as 

 is the case in Birds. 



There is no " inferior larynx " in any Mammal, and the upper 

 aperture of the true larynx is always protected by an epiglottis. 



The kidneys in Mammals are situated in the lumbar region, 

 and exhibit a division of their substance into cortical and 

 medullary portions. 



