160 ANIMAL BIOLOGY 



vegetables. Vitamin D in inadequate amounts results in rickets, a 

 disease characterized by various skeletal deformities. Oil from the 

 liver of the cod and halibut are rich sources, and the action of ultra- 

 violet rays on certain sterols produces the vitamin, so the exposure 

 of the human body to sunlight, within reasonable limits, is beneficial. 



F. Ductless Glands 



Finally, we must not overlook certain accessories of the ali- 

 mentary canal which lose all direct connection with it as develop- 

 ment proceeds — really glands that have carried, as it were, the 

 process of outpocketing from the digestive tract to the breaking 

 point and become ductless glands. Such, for instance, are the 

 thyroid and thymus glands near the anterior end of the esoph- 

 agus. The thymus in Man regresses during early childhood, 

 while the thyroid delivers its secretion, a hormone, directly into 

 the blood stream as an internal, or endocrine secretion. 

 We shall have occasion later to discuss the important coordinating 

 functions carried out by hormones secreted by ductless glands 

 and other endocrine organs. At the moment we may merely 

 remark that the pancreas is stimulated to secrete its digestive 

 enzymes by a hormone, known as secretin, brought to it by the 

 blood. Secretin is liberated into the blood by special gland cells 

 in the wall of the small intestine when food enters. (Fig. 110.) 



Certainly, at first glance, the complicated digestive system of the 

 Vertebrate may seem to have little in common with that of the 

 Earthworm, but as a matter of fact the fundamental plan is the 

 same. The differences which are present are chiefly the result of an 

 increase of the area of the alimentary canal, not only to afford 

 greater seeretive and absorptive surface and a larger variety 

 and amount of digestive substances, but also to prolong the length 

 of time the food is subjected to treatment. This increase in area 

 has been effected by folds and elevations of the inner surface of 

 the tract; by outpushings of limited areas of the tube to form large 

 glands which in most cases contribute their products to their point 

 of origin through ducts ; and by increasing the length of the inner 

 tube as compared with the outer tube, or body wall, which results 

 in throwing the intestine into various convolutions within the body 

 cavity. Thus is met the increasingly complex nutritional demands 

 of more highly organized animals. 



