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TEXT-BOOK OF PHYSIOLOGY 



ten to fifteen minutes after ligation of the blood-vessels of the limb or even 

 after its amputation, when the corresponding nerve is stimulated. 



As the sweat-glands are always in a state of more or less activity it is 

 assumed that the sweat centers possess a certain degree of tonicity, though 

 the cause for such tonicity has not been made apparent. It is quite possible 

 that the presence of carbon dioxid in the blood, as well as the temperature, 

 may be important factors, since an increase in the venosity of the blood or 

 a rise in temperature is usually followed by an increase in the amount of 

 sweat excreted. The centers may also be excited to increased activity by 

 both central and peripheral causes, e.g., psychic states of an emotional 

 character and a rise in the external temperature. Pilocarpin injected into 

 the blood causes a profuse secretion even when the nerves have been divided. 

 Its action is supposed to be exerted on the terminal branches of the nerves 



and possibly on the cells themselves. As in 

 the case of the salivary glands atropin sus- 

 pends the activity of the terminal branches of 

 secretor nerves. 



Hairs. Hairs are found in almost all 

 portions of the body, and can be divided 

 into 



1. Long, soft hairs, on the head. 



2. Short, stiff hairs, along the edges of the 

 eyelids and nostrils. 



3. Soft, downy hairs on the general cuta- 

 neous surface. 



They consist of a root and a shaft. The 

 shaft is oval in shape and about 60 micro- 

 millimeters in diameter; it consists of fibrous 

 tissue, covered externally by a layer of im- 

 FIG. 214. LARGE SEBACEOUS bricated cells, and internally by cells contain- 

 GLAND. r. Hair in its follicle. 23, m ~ granular and pigment material. 



4, c. Lobules of the gland. 6. Ex- & . . r . . , ,, , . ,, 



cretory duct traversed by the hair. The root of the hair is embedded in the 

 (Sappey.} hair-follicle, formed by a tubular depression 



of the skin, extending nearly through to the 



subcutaneous tissue; its walls are formed by the layers of the corium, covered 

 by epidermic cells. At the bottom of the follicle there is a papillary pro- 

 jection of amorphous matter, corresponding to a papilla of the true skin, 

 containing blood-vessels and nerves, upon which the hair-root rests. 

 The investments of the hair-roots are formed of epithelial cells, consti- 

 tuting the internal and external root-sheaths. 



The lower portion of the hair-follicle is connected with the upper surface 

 of the derma by bundles of non-striated muscle-fibers which are termed 

 arrectores pilorum muscles. Their inclination and insertion are such that 

 their contraction is followed by erection of the hair-follicle and hair-shaft. 

 These muscles are excited to action by nerves termed pilo-motor nerves. 



THE SEBUM 



The sebum or sebaceous matter is a peculiar oily material produced 

 by specialized glands in the skin. It consists of water, epithelium, pro- 

 teids, fat, cholesterin, and inorganic salts. 



