EXCRETION 473 



'the nerve system. Here, as in other secreting glands, the fluid is derived 

 from materials in the lymph-spaces, furnished by the blood. Generally 

 the two conditions, increased blood-flow and increased glandular action, 

 coexist. At times, however, a profuse clammy perspiration is secreted with 

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

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

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



The Sweat Nerves. The sweat nerves which excite the activities of the 

 epithelium of the sweat-glands have their origin in nerve-cells located in the 

 anterior and lateral gray matter of the spinal cord. The sweat nerves, like 

 ithe vaso-motor nerves, do not pass directly to the gland-cells, but indirectly 

 I by way of the ganglia of the sympathetic chain. In these ganglia the sweat 

 I nerves terminate, their end-branches aborizing around the nerve-cells. From 

 I the cells of these ganglia new nerve-fibers arise which then pass without 

 ! interruption to their final destination. The former are termed pre-gangli- 

 ! onic, the latter post-ganglionic. From their origin ^and distribution it is 

 apparent that the sweat nerves are constituent portions of the autonomic 



nerve system. 



From experiments made on animals and from observation of clinic con- 

 ditions in human beings, it may be stated in a general way, that the pre- 

 ganglionic fibers which emerge from the spinal cord in the ventral roots of 

 spinal nerves between the second thoracic and third lumbar nerves may be 

 1 divided into four groups, viz.: (i) Those distributed to the sweat-glands of 

 the skin of the trunk of the body; (2) those for the sweat-glands of the fore 

 limbs- (3) those for the sweat-glands of the head, neck and face; (4) those 

 for the sweat-glands of the hind limbs. These nerves pursue the same route 

 as the vaso-motor nerves with which they are associated in function (see 



The exact course for the sweat nerves has been experimentally deter- 

 mined only for the cat and dog. In these animals, however sweat-glands 

 are found only in the balls of the feet. According to Langley s observations 

 the sweat nerves for the forefeet leave the spinal cord ^in the thoracic 

 nerves from the fourth to the tenth inclusive. After passing into the sym- 

 pathetic chain they ascend to the stellate ganglion, around the cells of which 

 their end-branches arborize. From this ganglion non-mediated fibers pass 

 in the gray rami communicantes to the nerves composing -the bracmal plexus 

 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 me luded 

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

 the formation of the sacral plexus and through which ^^ <***; 



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

 probable, though its location has never been definitely determined. 



. 



