THE ^PANCREAS 263 



These are the organs which resemble glands but have no ducts. 

 They are supplied with sympathetic nerves, and possess many 

 lymphatics and blood-vessels; the secretions which they produce 

 are internal secretions and are carried in these vessels. The most 

 important ductless glands are the spleen, adrenal bodies and 

 certain portions of the pancreas, in the abdomen; thyroid, para- 

 thyroid, and ^thymus bodies in the neck; pituitary body (or 

 hypophysis) in the cranial cavity. To these may be added the 

 ovaries (also the carotid, parasympathetic and coccygeal bodies). 



The cells of the chromaffin tissues are found in the interior of 

 the adrenal bodies and in certain small groups which are ranged 

 along the abdoninal aorta. 



Each of the structures of the endocrin system bears a relation 

 to one or more of the others which is not yet perfectly understood. 



Their secretions have never been obtained for examination 

 but there is abundant evidence that they exist. 



The name autocoid substances has been proposed for the active 

 agents in these secretions. It is believed that there are two kinds 

 of autocoid substances: the hormones, which stimulate activity 

 in tissues to which the blood carries them, and the chalones, 

 which inhibit or prevent activity where they are carried. Each 

 member of the endocrin system has its own special autocoid 

 substance. 



So little is understood of the action of these various organs 

 that descriptions are necessarily brief. 



THE PANCREAS 



In addition to the digestive ferments of the pancreas it produces 

 another and highly important substance, which either disposes of 

 sugar in the blood, or is associated with the glycogenic function of 

 the liver, or both. This is supposed to be the special function of 

 groups of cells called "islands of Langerhans" which are embedded 

 in the substance of the pancreas. They resemble glands but 

 have no ducts; they are surrounded by a network of capillaries 

 and their internal secretion is transmitted by these vessels. 



The blood supply to the pancreas is very free, being derived 

 from the hepatic, splenic and superior mesenteric arteries. This 

 indicates the importance of the gland. 



