EXCRETION. 



489 



for the secretion can be excited and the flow maintained for a period of from 

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

 after its amputation, when the corresponding nerve is stimulated. 



The sweat-glands may be excited to activity by their related nerve-centers, 

 either by central, by reflex, or by local peripheral influences. Among the first 

 may be mentioned mental emotions, venosity of the blood, increased tempera- 

 ture of the blood, hot drinks, violent muscular exercise, etc. Among the 

 second may be mentioned powerful stimulation of various afferent or sensor 

 nerves, heightened external temperature, etc. Among the last may be 

 mentioned various drugs. 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 suspends the activity 

 of the terminal branches of the 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- 

 bricated cells, and internally by cells contain- 

 ing granular and pigment material. 



nni_ , < , i i_ i_ j j j ,1 



Cheroot of the hair is embedded in the 

 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. 



FIG. 226. LARGE SEBACEOUS 

 GLAND, i. Hair in its follicle. 2,3, 



4, <. Lobules of the eland. 6. Ex- 



et ory duct traversed by the hair. 



