650 MANUAL OF ZOOLOGY. 



presents the "dentition" of both jaws in a condensed and 

 easily recognised form. 



According to Owen, the typical permanent dentition of a 

 diphyodont Mammal would be expressed by the following 

 formula : 



33 i i 44 33 



The four kinds of teeth are indicated in such a formula by the 

 letters incisors /', canines c, praemolars /;;*, molars m. The 

 numbers in the upper line indicate the teeth in the upper jaw, 

 those in the lower line stand for those in the lower jaw ; and 

 the number of teeth on each side of the jaw is indicated by 

 the short dashes between the figures. 



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 Verte- 

 brate 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 cavse 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 



