EXCRETION 477 



The Sweat-glands. These glands are tubular in shape, the inner 

 extremity of each being coiled upon itself a number of times, forming a 

 little ball situated in the derma or the subcutaneous connective tissue. From 

 this coil the duct passes up in a straight direction to the epidermis, where 

 it makes a few spiral turns, after which it opens obliquely on the surface. 

 The gland consists of a basement membrane lined with epithelial cells. 

 It is supplied abundantly with blood-vessels and nerves. The sweat- 

 glands are extremely numerous all over the cutaneous surface, though 

 they are more thickly disposed in some situations than others. They 

 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 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 

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

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

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

 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 

 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 

 forlhe 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 



page 377)- 



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-medullated fibers pass 

 in the gray rami communicantes to the nerves composing the brachial plexus 



