238 PHYSIOLOGY FOR DENTAL STUDENTS. 



dog. Following such stimulation drops of sweat are found on 

 the paw. The secretion is not due to an increased blood flow, as 

 can be shown by stimulating the nerves in a limb severed from 

 its blood supply, in which case a few drops of sweat will still 

 appear. A center in the brain and subsidiary centers in the 

 spinal cord have been found which, when stimulated, produce 

 a secretion of sweat. 



Some drugs have the peculiar action of exciting the secretion 

 of sweat, either reflexly through the nerve center or by stimula- 

 tion of the nerve endings about the cells of the glands. To the 

 former class belong such drugs as strychnine and picrotoxin, and 

 to the latter, pilocarpin. Atropin, on the other hand, inhibits 

 the secretion by paralyzing the secretory nerve mechanism. An 

 increase in the external temperature will cause a secretion of 

 sweat only when the sensory and motor nerves of the part are 

 both functional. To stimulate the sweat nerves, heat therefore 

 must act reflexly through the sensory nerves and the centers of 

 the brain or spinal cord. 



THE SEBACEOUS GLANDS. Besides the sweat glands there are 

 numerous other glands in the skin. These are associated with 

 the hairs, and are called sebaceous glands. They secrete an oily 

 semiliquid material which affords protection to the hair and the 

 skin. Its oily nature prevents the hair from becoming too brittle, 

 and protects the skin from moisture. 



THE SECRETION OF MILK. The mammary glands are modified 

 sebaceous glands which secrete a nutrient fluid, milk. The 

 glands are much better developed in the female than in the male, 

 and are excited to physiological activity ait the birth of the child. 

 Human milk is a white or yellowish fluid, without odor and with 

 a peculiar sweet taste. It contains protein substances called 

 caseinogen, lact-albumin, and lact-globulin ; also a sugar called 

 lactose or milk sugar, and fats and inorganic matter, as the chlo- 

 rides of sodium, potassium and calcium. Human milk is by far 

 the best food for the infant, and should be replaced by other 

 food only when absolutely necessary. 



