4 88 TEXT-BOOK OF PHYSIOLOGY. 



probably average 400 to the square centimeter; the total number has been 

 estimated at from 2,000,000 to 2,500,000. 



The Influence of the Nerve System on the Production of Sweat. 

 The secretion of sweat, though a product of the activity of epithelial cells 

 and dependent on a variety of conditions, is regulated to a large extent by 

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

 secretor 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 location for 

 the different regions of the body have not yet been satisfactorily determined. 

 From observation of clinic and pathologic conditions in human beings and 

 from experiments made on animals it may ^be stated in a general way 

 that the centers for the head and face lie in the upper thoracic region of the 

 cord; for the upper extremities, in the upper two-thirds of the thoracic 

 region; for the lower extremities, in the lower thoracic and upper lumbar 

 region. The secretor nerves which emerge from these centers are contained 

 in the ventral roots of the thoracic and upper lumbar nerves, which they leave 

 by way of the white rami communicantes as medullated (pre-ganglionic) fibers 

 to enter the sympathetic ganglia, around the cells of which they arborize. 

 From these ganglia non-medullated (post-ganglionic) fibers emerge, re-enter 

 the spinal nerves, with the exception of those for the head and face, and 

 then pass to the sweat glands in various regions of the body, following a 

 course similar to that pursued by the vaso-constrictor nerves for correspond- 

 ing regions. It is probable, though it has not been demonstrated, that 

 there is also in the medulla a general dominating sweat center. 



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 fore-feet 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-medullated fibers pass 

 in the gray rami communicantes to the nerves composing the brachial 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 non-medullated 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. 



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 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; 



