EXCRETION. 



437 



creased blood-flow and increased glandular action, coexist. At 

 times, however, a profuse clammy perspiration is secreted with dimin- 

 ished blood-flow. Two sets of nerves are evidently concerned in this 

 process: viz., vaso-motor nerves, which regulate the blood-supply, and 

 secretory nerves, which stimulate the gland-cells to activity. 



The nerve-centers which control the sweat-glands are situated in 

 the spinal cord, though the number of such centers and their exact 

 l6cation for the different regions of the body have not yet been 

 satisfactorily determined. In a general way it may be stated that the 

 centers for the head and face lie in the upper cervical portion of the 

 cord; for the upper extremities, in the lower cervical portion; for the 

 lower extremities, in the lower dorsa.1 and upper lumbar portion. 

 The secretory nerves which emerge from these centers reach the glands 

 of the face and head through the cervical sympathetic; of the arms 

 and legs, through the brachial plexus and the sciatic nerves. It is 

 probable that there is also a general dominating sweat center located 

 in the medulla oblongata. 



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

 is shown by the fact that stimulation of the peripheral end 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 conditions 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 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, reflex, or peripheral influences. Among 

 the first may be mentioned mental emotions, venosity of the blood, 

 increased temperature of the blood, hot drinks, violent muscular 

 exercise, etc. Among the second may be mentioned powerful 

 stimulation of various afferent or sensory* 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 secretory 

 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. 



