THE SKIN 321 



the centers in the medulla and the cord. The least normal, a watery 

 state of the blood, probably produces its effect directly on the epithelium 

 of the sweat-glands, much as does the same sort of action in the kidneys. 

 In just what manner the presence of too much carbon dioxide in the blood 

 works is not at present understood. Many sorts of reflex stimulation of 

 the spinal cord and brain cause an increased flow of sweat, whether the 

 stimuli come from the sensory nerves or from deeply lying parts of the 

 brain such as the optic thalami, the centers of emotional expression. 

 The sweat-glands are not readily stimulated directly by warming the 

 skin containing them, but easily and from many different directions by 

 first stimulating the controlling centers in the central nervous system. 



We have already considered above in two different places the two 

 general functions of the sweat, namely, to excrete water (and many other 

 things in small amounts) and, largely by this latter means, to regulate the 

 loss of body-heat. These statements need not be renewed here. As a 

 means of excreting water from the organism the sweat-glands seem to be 

 just about on a parity with the kidneys. As regulator of heat-loss the 

 sweat is paramount. 



The Secretion of Sebum is a dermal function more or less closely 

 concerned in the skin's protection of the body. Because largely useful, 

 sebum is classed as a secretion rather than as an excretion. Sebum is 

 of many different varieties according to its precise function on different 

 parts of the body. It varies in the hair-follicles, in the external auditory 

 meatus, on the edges of the eye-lids, on the lips and in corners of the 

 mouth, in the cervix, corona gland is, foreskin, labia minora, etc. Vernix 

 caseosa, the sebum covering the body at birth, is still unlike all the others. 



Wherever it occurs, the essence of sebum's composition is fat, largely 

 olein and palmitin. These make the excretion liquid when secreted 

 and allow it to harden more or less on exposure to the cool air. Soaps 

 are present in considerable proportion, and glycerin, and a casein-like 

 nucleo-proteid. It contains earthy and alkaline phosphates and chlorides 

 of sodium and potassium. Cholesterin is always to be found. In 

 appearance also sebum varies largely with the place where it is secreted ; 

 so does its odor as well, smegma, for example, having in the brutes an 

 odor with much biological value. 



The sebaceous glands are attached to the follicles of the hairs and pour 

 their product into them, whence it is distributed throughout the epidermis 

 by the movements of the skin. Just within the hair-sheath is the mem- 

 brana propria, continuous with a similar layer in the follicle. Within 

 this in turn are several strata of gland-cells proper, the outer two or three 

 being like those of the external root-sheath of the hairs. The inner 

 strata of epithelial cells are different, containing fat-globules in quantities 

 sufficient to give the cytoplasm a reticulated appearance, while the nuclei 

 are compressed. These fat-globules constitute the product of the 

 secretion, for the innermost cells containing them are continually break- 

 ing down in a mass, thus becoming sebum. These .are as continually 

 replaced by the basilar cells below them, which in turn develop sebum- 

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