Integrative Systems 215 



exists where it is advantageous to the animal to have it. Quick and 

 vigorous action is required of somatic muscles. They are accordingly 

 striated, and it is obviously to the animal's advantage that they be 

 under voluntary control (unless it can be shown that it would be better 

 if the animal-as-a- whole were an absolute automaton). Unquestionably 

 it is to the animal's advantage that its internal visceral activities be 

 under autonomic control. But where there is local need of high muscu- 

 lar efficiency, as in the heart, the visceral muscle becomes striated. In 

 doing so, it does not thereby become voluntary. 



II. Endocrinal Organs 



The secreting of hormones is carried on by several organs which 

 are exclusively devoted to that function. In other organs, whose pri- 

 mary functions may be concerned with such processes as digestion or 

 reproduction, the secreting of endocrinal substances may occur as an 

 incidental or secondary function. Specialized endocrinal organs are 

 of various and most unlike origin. An important group, including the 

 thyroid, parathyroid, and thymus glands, arises by outgrowth 

 of endodermal pouches from the embryonic pharynx. Two glands 

 develop by outgrowth from the wall of the brain — a pituitary lobe 

 from the floor, and the pineal gland from the roof, of the dien- 

 cephalon. Certain important glands in the trunk region have common 

 origin with the autonomic ganglions. All of the three basic embry- 

 onic tissues or "germ layers" are represented among the endocrinal 

 organs. Some come from endoderm via the digestive tube, some 

 from ectoderm via the neural tube, and some develop from local 

 mesoderm. 



The English physiologists, Bayliss and Starling, discovered that, 

 when the acid contents of the stomach enter the intestine, the diges- 

 tive secretion of the pancreas is poured into the intestine. This happens 

 even when all nervous connections between the organs have been cut. 

 They concluded that a "chemical messenger" or hormone must be 

 produced by epithelial cells lining the duodenum and carried to the 

 pancreas by the blood. They gave to this hypothetic substance the 

 name "secretin." The duodenum, primarily a digestive region of the 

 intestine, is secondarily an endocrinal organ. 



Pancreas 



The pancreas has an endocrinal as well as a digestive function. 

 Scattered among lobules of the pancreas are aggregations of peculiar 

 cells, the "pancreatic islands" or islands of Langerhans (Fig. 184). 

 These "islands" secrete a hormone, insulin. Carried by the blood, 

 it regulates the oxidation of carbohydrates in the tissues and the stor- 



