404 GENERAL ZOOLOGY 



just anterior to the kidneys, the thymus (Fig. 213, 17), and the 

 thyroid gland (Fig. 213, 18). Their functions are not thor- 

 oughly understood. 



The Circulatory, Respiratory, and Excretory Systems. The 

 heart is inclosed in a pericardium (Fig. 213, 20), and is of the 

 four-chambered type found in the birds. There is a complete 

 double circulation of the blood in the squirrel, as in birds. 

 The lymphatic vessels of the abdomen, called lacteals, which 

 carry from the intestine the absorbed fatty materials of food, 

 unite to form a thoracic duct, which extends anteriorly and 

 empties into the venous system near the heart. 



At the anterior end of the larynx is a cartilaginous flap 

 called the epiglottis (Fig. 213, 30). The lungs (Fig. 213, 32) 

 are larger and more extensible than in the birds, and hang 

 free in the thoracic cavity. Respiration is effected mainly by 

 movements of the diaphragm and the ribs, thus altering the 

 size of the thoracic cavity and causing air to enter and to 

 leave the lungs. 



The kidneys (Fig. 21.3, 33) are almond-shaped bodies in the 

 dorsal part of the abdominal cavity. The ureters (Fig. 213, 34) 

 lead from them to the urinary bladder (Fig. 213, 35), whence 

 the waste products are carried to the surface by the urethra 



(Fig. 213, 36, 37). 



The Skeletal System. The skeleton is, in general, built 

 upon the plan with which we have become familiar in the 

 study of the frog, the lizard, and the pigeon. The cranium 

 (Fig. 213, 38) is articulated to the vertebra by two condyles, 

 as in the amphibians. All the vertebra (Fig. 213, 40), except 

 those in the pelvic region, are free. 



The Nervous and Muscular Systems. The nervous system 

 is similar to that of the bird, but the cerebrum (Fig. 213, 44) 

 is considerably more differentiated. The muscles, especially 

 those in the hind legs, form a complex system adapted to 

 strong and rapid movement. 



