SECRETIONS OF THE SKIN 39 



yellow color, alkaline in reaction, viscid, and coagulable by 

 heat. 



Lacrymal Glands. The conclusion reached for the salivary 

 glands may be applied with little alteration to the glands of 

 the nasal mucous membrane and to the lacrymal glands. The 

 latter resemble an albuminous salivary gland, receiving cranial 

 secretory fibers by way of the fifth nerve, and sympathetic 

 fibers by way of the cervical sympathetic. Stimulation of 

 most sensory nerves produces a secretion reflexly. The ducts 

 of the gland lead to the conjunctiva of the upper eyelid, and 

 usually the secretion is just sufficient to keep the eye moist, 

 and is drained into the nasal cavity by way of the lacrymal 

 duct. When the secretion is formed in superabundance, it 

 appears as tears. These are alkaline in reaction and contain 

 1 per cent, of solids, chiefly chloride of sodium. 



Secretions of the Skin. Perspiration. The perspiration is 

 a colorless liquid with a peculiar odor, a salty taste, an acid 

 reaction, and a specific gravity of 1004. The amount formed 

 varies enormously with the temperature, with exercise, with 

 psychical and pathological conditions, but may be put at an 

 average of from 700 to 900 grams a day, a little more than 

 half the total of urine excreted. Its constituents are water, 

 inorganic salts, traces of fat, fatty acids, cholesterin, and urea. 

 Sodium chloride forms from 2 to 3.5 parts in a thousand. 

 The urea of perspiration in determinations of destroyed pro- 

 teins is usually neglected. During extraordinary muscular 

 work the nitrogen eliminated by the skin amounts to 0.7 or 

 0.8 gram. Sweat glands are found over the entire surface of 

 the skin, with the exception of the external ear. Their total 

 number is about 2,000,000. 



Insensible perspiration increases in quantity with increase 

 of temperature until a certain critical point is reached, when 

 it is markedly increased and appears as visible sweat. The per- 

 centage of carbon dioxide is increased at the same time. 

 Normally the glands are stimulated by exercise and high 

 temperature. In the latter case it is produced through the 

 central nervous system. Instead of acting on the glands 

 directly, the heat affects cutaneous sensory nerves and reflexly 

 excites the glands. Sweating can be brought about by stimu- 

 lation of the afferent nerves and by dyspnea; by the latter, 



