KIDNEY AND SKIN AS EXCRETORY ORGANS. 871 



Histologically and anatomically the arrangement of the sweat 

 fibers resembles that of the vasoconstrictor fibers, and, reasoning 

 from analogy, one might suppose the existence of a general sweat 

 center in the medulla comparable to the vasoconstrictor center, 

 but positive evidence of the existence of such an arrangement is 

 lacking. It has been shown that when the medulla is separated 

 from the cord by a section in the cervical or thoracic region the 

 action of dyspnea, or of various sudorific drugs supposed to act on 

 the central nervous system, may still cause a secretion. On the 

 evidence of results of this character it is assumed that there are spinal 

 sweat centers; but whether these are few in number or represent- 

 simply the various nuclei of origin of the fibers to different regions 

 is not definitely known; It is possible that in addition to these 

 spinal centers there is a general regulating center in the medulla. 

 Sebaceous Secretion. The sebaceous glands are simple or 

 compound alveolar glands found over the cutaneous surface, usually 

 in association with the hairs, although in some cases they occur 

 separately, as, for instance, on the prepuce and glans penis, and 

 on the lips. When they occur with the hairs the short duct opens 

 into the hair follicle, so that the secretion is passed out upon the 

 hair near the point at which it projects from the skin. The alveoli are 

 filled with cuboidal or polygonal epithelial cells, which are arranged 

 in several layers. Those nearest the lumen of the gland are filled 

 with fatty material. These cells are supposed to be cast off bodily, 

 their detritus going to form the secretion. New cells are formed 

 from the layer nearest the basement membrane, and thus the glands 

 continue to produce a slow but continuous secretion. The sebaceous 

 secretion, or sebum, is an oily, semiliquid material that sets, upon 

 exposure to the air, to a cheesy mass, as is seen in the comedones 

 or pimples which so frequently occur upon the skin from occlusion 

 of the opening of the ducts. The exact composition of the secretion 

 is not known. It contains fats and soaps, some cholesterin, albu- 

 minous material (part of which is a nucleo-albumin often described 

 as a casein), remnants of epithelial cells, and inorganic salts. The 

 cholesterin occurs in combination with a fatty acid, and is found in 

 especially large quantities hi sheep's wool, from which it is extracted 

 and used commercially under the name of lanolin. The sebaceous 

 secretion from different places, or in different animals, is probably 

 somewhat variable hi composition as well as in quantity. The 

 secretion of the prepuce is known as the smegma prceputii; that of 

 the external auditory meatus, mixed with the secretion of the neigh- 

 boring sweat glands or ceruminous glands, forms the well-known 

 earwax or cerumen. The secretion in this place contains a reddish 

 pigment of a bitterish-sweet taste, the composition of which has 

 not been investigated. Upon the skin of the newly born the se- 



