408 GENERAL ZOOLOGY 



anterior portion, the small intestine, can be distinguished 

 from the posterior portion, or rectum. At the junction of 

 the small intestine and rectum is the cxcum, from which 

 projects a closed finger-like vermiform appendix. 



Ductless Glands. Several glands, called ductless glands 

 (owing to the absence of a duct leading from them), are 

 present in vertebrates. With the single exception of the 

 spleen (mentioned in each case in the course of the state- 

 ments concerning the digestive system) they have not been 

 referred to in our brief discussion of the internal anatomy 

 of the classes of vertebrates, but as they show plainly in the 

 squirrel attention may be called to them at this time. They 

 are, besides the spleen (Fig. 215), the adrenal capsules, just 

 anterior to the kidneys, the thymus gland and the thyroid 

 gland. In addition to these, there are other ductless glands 

 of much smaller size. Among these are the parathyroids, 

 closely associated with the thyroid, and the hypophysis in 

 the ventral part of the brain. Groups of cells in the gon- 

 ads, called the interstitial cells, have nothing to do with the 

 formation of germ cells but regulate the development of the 

 secondary sexual characters. Though their functions are 

 but imperfectly known the ductless glands are coming to be 

 considered among the most important organs of the body. 

 They regulate and control the actions of many other organs. 



The Circulatory System. The heart is inclosed in a pericar- 

 dium (Fig. 215) 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. 



The Respiratory System. At the anterior end of the larynx 

 is a cartilaginous flap called the epiglottis (Fig. 215). The 



