768 PHYSIOLOGY OF DIGESTION AND SECRETION. 



new secretion. The secretion so formed is thin and limpid, and has 

 a marked alkaline reaction. The anatomical course of these fibers 

 has been worked out in the cat with great care by Langley.* He 

 finds that for the hind feet they leave the spinal cord chiefly in the 

 first and second lumbar nerves, enter the sympathetic chain, and 

 emerge from this as postganglionic fibers in the gray rami, proceed- 

 ing from the sixth lumbar to the second sacral ganglion, but chiefly 

 in the seventh lumbar and first sacral, and then join the nerves of 

 the sciatic plexus. For the forefeet the fibers leave the spinal cord 

 in the fourth to the tenth thoracic nerves, enter the sympathetic 

 chain, pass upward to the first thoracic ganglion, whence they are 

 continued as postganglionic fibers that pass out of this ganglion by 

 the gray rami communicating with the nerves forming the brachial 

 plexus. The action of the nerve fibers upon the sweat glands can 

 not be explained as an indirect effect, for instance, as a result of 

 a variation in the blood-flow. Experiments have repeatedly shown 

 that, in the cat, stimulation of the sciatic still calls forth a secre- 

 tion after the blood has been shut off from the leg by ligation of 

 the aorta, or indeed after the leg has been amputated for as long 

 as twenty minutes. So in human beings it is known that profuse 

 sweating may often accompany a pallid skin, as in terror or 

 nausea, while, on the other hand, the flushed skin of fever is char- 

 acterized by the absence of perspiration. There seems to be no 

 doubt that the sweat nerves are genuine secretory fibers, causing 

 a secretion in consequence of a direct action on the cells of the sweat 

 glands. In accordance with this physiological fact histological 

 work has demonstrated that special nerve fibers are supplied to 

 the glandular epithelium. According to Arnstein, the terminal 

 fibers form a small, branching, varicose ending in contact with the 

 epithelial cells. The sweat gland may be made to secrete in many 

 ways other than by direct artificial excitation of the sweat fibers, 

 for example, by external heat, dyspnea, muscular exercise, strong 

 emotions, and by the action of various drugs, such as pilocarpin, 

 muscarin, strychnin, nicotin, picrotoxin, and physostigmin. In all 

 such cases the effect is supposed to result from an action on the 

 sweat fibers, either directly on their terminations or indirectly upon 

 their cells of origin in the central nervous system. In ordinary 

 life the usual cause of profuse sweating is a high external temper- 

 ature or muscular exercise. With regard to the former it is known 

 that the high temperature does not excite the sweat glands im- 

 mediately, but through the intervention of the central nervous 

 system. If the nerves going to a limb be cut, exposure of that 

 limb to a high temperature does not cause a secretion, showing 

 that the temperature change alone is not sufficient to excite the 

 *" Journal of Physiology," 12, 347, 1891. 



