MAMMALIA. 



289 



the molars and premolars. The hard tongue, working against 

 the ridged palate, assists in this process of mastication. Saliva 

 is poured by the salivary glands into the mouth-cavity, and not 

 only lubricates the food, but also, in virtue of a ferment, ptyalin, 

 which it contains, converts more or less of the starch into soluble 

 grape-sugar. The gastric juice, bile, and pancreatic juice act as in 

 preceding cases (p. 194). 



5. Circulatory Organs. Well developed blood and lymph systems 

 are present. 



(1) Blood System. The blood consists of liquid plasma, in 

 which are suspended colourless corpuscles of the usual type, and 



Bile-capillary. Nucleus of a liver-cell. 



Bile- capillary divided. Bile-capillary. 



Fig. 86. HISTOLOGY OF RABBIT'S LIVER (after Hering). Relation of 

 liver-cells, bile-capillaries, and blood-capillaries. 



a much larger number of smaller red corpuscles, which are bicon- 

 cave, non-nucleated discs. The blood-containing tubes comprise 

 heart, arteries, veins, and capillaries. 



The conical heart (Fig. 85) enclosed in its pericardium, lies 

 between the lungs on the ventral side of the thoracic cavity, with 

 its apex directed backwards. It contains four chambers, two 

 thin-walled auricles, and two thick-walled ventricles. 



The broad base of the heart is chiefly made up of the dark, thin-walled 

 right and left auricles, to each of which is attached a plaited fold, the 

 auricular appendix, while the rest of the heart is made up by the firm 

 2 19 



