200 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



blood has been shut off from the leg by ligatiou 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 

 characterized by the absence of perspiration. There seems to be no doubt 

 at all that the sweat-nerves are genuine secretory fibres, producing the secre- 

 tion directly by their action on the cells of the sweat-glands. In accordance 

 with this physiological fact recent histological work has demonstrated that 

 special nerve-fibres are supplied to the glandular epithelium. According to 

 Arnstein * the terminal fibres form a small branching varicose ending in con- 

 tact with the epithelial cells. The sweat-gland may be made to secrete in 

 many ways other than by direct artificial excitation of the sweat-fibres ; for 

 example, by external heat, dyspnoea, 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-fibres, 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 immediately, 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 gland or its terminal nerve-fibres. We must suppose, therefore, 

 that the high temperature acts upon the sensory cutaneous nerves, possibly 

 the heat-fibres, and reflexly stimulates the sweat-fibres. Although external 

 temperature does not directly excite the glands, it should be stated that it 

 affects their irritability either by direct action on the gland-cells or, as is more 

 likely, upon the terminal nerve-fibres. At a sufficiently low temperature the 

 cat's paw does not secrete at all, and the irritability of the glands is increased 

 by a rise of temperature up to about 45 C. 



Dyspnoea, muscular exercise, emotions, and many drugs affect the secretion, 

 probably by action on the nerve-centres. Pilocarpin, on the contrary, is 

 known to stimulate the endings of the nerve-fibres in the glands, while atropin 

 has the opposite effect, completely paralyzing the secretory fibres. 



Sweat-centres in the Central Nervous System. The fact that secretion of 

 sweat may be occasioned by stimulation of afferent nerves or by direct action 

 upon the central nervous system, as in the case of dyspnoea, implies the exist- 

 ence of physiological centres controlling the secretory fibres. The precise loca- 

 tion of the sweat-centre or centres has not, however, been satisfactorily deter- 

 mined. Histologically and anatomically the arrangement of the sweat-fibres 

 resembles that of the vaso-constrictor fibres, and, reasoning from analogy, one 

 might suppose the existence of a general sweat-centre in the medulla compara- 

 ble to the vaso-constrictor centre, but positive evidence of the existence of such 

 1 Analomischer Anzeiger, 1895, Bd. x. 



