478 TEXT-BOOK OF PHYSIOLOGY 



and then to the feet. The sweat nerves for the hind feet leave the cord 

 mainly in the first and second lumbar and terminate in sympathetic ganglia, 

 from which the post-ganglionic nerves pass into the nerve-trunks included 

 between the sixth lumbar and the second sacral nerves, which enter into 

 the formation of the sacral plexus and through which they pass to the feet. 

 The existence of a dominating sweat center in the medulla oblongata is 

 probable, though its location has never been definitely determined. 



That the sweat-glands are stimulated to activity by nerve impulses is 

 shown by the fact that stimulation of the peripheral ends of the divided 

 cervical sympathetic, of the brachial plexus, or of the sciatic nerve is followed 

 in a few seconds by a profuse secretion. Though under physiologic con- 

 ditions there is a simultaneous dilatation of the blood-vessels and an increased 



supply of blood, this is merely a condition 

 and not a cause of the secretion; for the 

 secretion can be excited and the flow main- 

 tained 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. 



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 



3 '^^wu 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 tem- 

 FIG. 2ii. LARGE SEBACEOUS perature is usually followed by an increase 

 GLAND, i Hair in its follicle 2, 3, m the amount of sweat excreted. The cen- 

 4- 5- Lobules of the gland. 6. Excre- j . , , . ,. ., 



tory duct traversed by the hair. ters may also be excited to increased activity 

 (Sappey.) 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 suspends 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 cutaneous 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 imbricated cells, and internally by cells containing 

 granular and pigment material. 



The root 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. 



