COORDINATION 



435 



creas, duodenum, thyroid, parathyroids, 

 adrenals, and pituitary. Although they are 

 referred to as ductless glands, the first 

 two do have ducts, although the endocrine 

 secretion does not leave the gland through 

 the ducts but instead enters tlie blood 

 stream directly. These two, together with 

 the duodenum have two separate glandular 

 functions. The gonads function in the pro- 

 duction of eggs and sperms in addition to 

 their endocrine function of producing hor- 

 mones that are responsible for tlie sec- 

 ondary sexual characteristics. The pancreas 

 produces digestive enzymes in addition to 

 insulin, and the duodenum has several 

 functions besides producing secretin. More- 

 over, a single gland such as the pituitary 

 produces several hormones, each with a 

 strikingly different function. Other endo- 

 crine glands may be found, although this 

 appears much less likely than the possibility 

 of discovering new functions for the glands 

 already known. 



From the very beginning of experimental 

 work in endocrinology, as this phase of 

 biology is called, the techniques of discov- 

 ering the function of a suspected gland 

 have been much the same. The gland is re- 

 moved. If changes appear in the animal's 

 normal function and if these can be re- 

 versed by replacing the gland or its extract, 

 then it is reasonably certain that the gland 

 has an endocrine function. A more com- 

 plete understanding can be had by using 

 extracts, for if the gland has multiple 

 functions, as in the case of the pituitary, 

 various fractions of the extract can be used 

 and these separate functions more clearly 

 defined. Sometimes it is desirable to inject 

 extracts into the normal animal to study 

 the possible effects of oversupply of a hor- 

 mone. In human beings, the occasional 

 dysfunctioning of endocrine glands because 

 of tumors or other abnormalities has given 

 physicians and biologists abundant material 

 to study and, in some cases, to conduct ex- 

 periments. Results of experimentation on 

 lower animals have also revealed a great 



deal of information that has been applied 

 directly to the alleviation of many endo- 

 crine aberrations in man. 



The ultimate goal in experimentation 

 with endocrines is to find the exact chemi- 

 cals that are involved and to be able to 

 prepare them in tlie laboratory. Once the 

 chemical structure of these substances is 

 known, they can be synthesized and used in 

 animals with deficiencies to restore normal 

 conditions. This has been accomplished for 

 a few hormones but there is still much to be 

 learned. Before this book comes off the 

 press a new endocrine gland may well have 

 been discovered or a new function attrib- . 

 uted to one already known. We will briefly 

 survey the seven that are fairly well known. 



The duodenum 



Two British physiologists, Bayliss and 

 Starling, found that the highly acidic food 

 passing from the stomach into the duode- 

 num stimulated the walls of the latter organ 

 to produce a substance which circulated in 

 the blood stream to the pancreas, causing 

 it to secrete its products into the digestive 

 tract. This substance they called secretin, 

 designating it also by the name hormone, 

 a name which has since come into general 

 usage for this class of substances. We shall 

 learn more about secretin in the chapter on 

 digestion. With their pioneer work, the field 

 of endocrinology was initiated. 



Pancreas 



This compound gland has ducts and its 

 most obvious function is that of producing 

 digestive enzymes which are drained off to 

 the duodenum through the pancreatic duct. 

 The digestive function of the pancreas will 

 be discussed in a later section; here we are 

 concerned only with the portion of the pan- 

 creas that produces the hormone insulin. 



The story of the discovery of insulin is 

 one of the more fascinating sagas in the 

 annals of biological science. As far back as 

 1890 it was known that if the pancreas were 

 removed from a dog, death followed in a 



