416 THE SKIN AND ITS APPENDAGES. 



pigment of black hair is not a derivative of blood-pigment, as it is 

 impossible to obtain either hsemopyrrol or a hsematinic acid on 

 appropriate treatment of the isolated product, viz., substances 

 which must be regarded as characteristic reduction-, viz., oxidation-, 

 products of the blood-pigment. On oxidation of the black pig- 

 ment of horse-hair he obtained a substance of the composition 

 C n H 22 O 2 , which seems to be identical with Butlerow's methy-dibutyl- 

 acetic acid. The elementary composition of the black pigment 

 obtained from different sources differs somewhat ; that derived from 

 black horse-hair had the composition C^K^NgSC^, another from 

 black sheep wool C^HggNgSO^. 



From white hair of horses and sheep he obtained white pigments 

 of the composition C 45 H 78 N 10 SO 20 and C 61 H 98 N 10 SO 20 . These white 

 pigments apparently represent the chromogens of the corresponding 

 black pigments. 



Into the various pigments which have been found in the skin of 

 reptiles, in the scales of fishes, in the feathers of birds, etc., it is 

 scarcely necessary to enter at this place. They partly belong to the 

 melanins, partly to the lipochromes ; others are classified as mela- 

 noids, while still others are closely related to the haemoglobins. To 

 a certain extent, moreover, the colors of birds' feathers appear to be of 

 a physical nature and referable to certain phenomena of interference. 



In the invertebrate animals various pigments are also observed, 

 but are for the most part unknown. The keratin, as I have already 

 stated, is here represented by other tegumentary substances, such as 

 chitin, tunicin, the hyalogens, and the skeletins (which see). 



The Sweat. The sweat is the specific, secretory product of the 

 corresponding glands, which are found imbedded in the lower portion 

 of the dermis. In man, these glands rank next in order to the 

 kidneys in importance as excretory organs of water, and are capa- 

 ble, to a certain extent at least, of assuming a vicarious activity 

 when the kidneys are diseased. In man their number is quite 

 large, exceeding 2,000,000. Their distribution, however, is not 

 uniform, and as a result there are certain portions of the body 

 in which a more abundant secretion is noted than in others. Such 

 regions are the forehead, the armpits, the palms of the hands, the 

 soles of the feet, etc. 



Among the mammalian animals, however, some exist in which 

 a secretion of sweat does not occur. This is the case in many 

 of the rodents and the goat. The sheep, the horse, and the apes, 

 on the other hand, sweat over their entire body, while other animals, 

 like the cat and the dog, sweat only from the balls of the toes. 



The amount of sweat excreted in man is very variable. It differs 

 in different individuals ; and is dependent upon the surrounding 

 temperature, the amount of water ingested, the temperature of the 

 body, the amount of exercise taken, etc. It is manifestly under the 

 control of the central nervous system, and is increased by painful 



