312 ANIMAL BIOCHEMISTRY 



Sweat contains about 99 per cent water. The main solid constituent 

 is sodium chloride, although small amounts of other inorganic ions 

 such as phosphates and sulfates are present. Urea is present in very 

 low concentrations along with traces of lactate, uric acid, creatine, and 

 volatile organic sulfur compounds. 



The skin also contains sebaceous glands at the roots of the hair. 

 These glands provide an oily secretion which prevents brittleness of 

 hair. 



Glands of Internal Secretion 



Ductless glands, also called endocrine glands or endocrine organs, 

 are unique in that they manufacture and secrete into the blood stream 

 chemical substances necessary for the stimulation and regulation of 

 other organs or metabolic processes. The chemical constituents re- 

 leased are called hormones. Their roles in metabolism are similar to 

 those of the vitamins (Chapter 14) with the only real differentiation 

 based on the fact that vitamins must be obtained from external sources 

 whereas hormones are produced in situ. Hormones, as is true of vita- 

 rains, vary extremely in complexity from rather simple compounds 

 like adrenaline and thyroxine to sterols and proteins. Many of the 

 biologically active endocrine secretions remain to be chemically iden- 

 tified. 



Gasfro/ntestina/ fiormones. The hormones produced in the diges- 

 tive tract play an active role in mobilizing the forces required for 

 digestion of foods (Chapter 17). Gastrin, produced by the pyloric 

 mucosa in response to stimulation by certain nitrogenous constituents 

 of food entering the stomach, stimulates the active hydrochloric acid 

 secretion from the fundic cells. Histamine functions in the stimu- 

 lation of the flow of gastric juices. But there is good evidence that 

 gastrin and histamine are synonymous. 



When hydrochloric acid of the acid chyme reaches the small intes- 

 tine, cells of the intestinal mucosa are stimulated to produce the 

 hormone secretin. This hormone is carried by the blood to the 

 pancreas, where it stimulates the flow of pancreatic juice containing 

 sodiimi bicarbonate to neutralize the chyme. Secretin has been ob- 

 tained in crystalline form. It is a polypeptide of unknown structure. 



Simultaneously a second hormone, pancreozymin, stimidates the 

 production of the pancreatic enzymes (page 434). Pancreozymin is 

 also polypeptide in nature. It can be separated from an alcohol solu- 

 tion also containing secretin by saturation with sodium chloride. A 

 third hormone, cnterogastrone, also originating in the intestinal 



