FUNCTIONS OF THE EXTERNAL INTEGUMENT. 525 



provided with ciliated secretory cells. The internal (ciliated) excretory duct opens 

 into the pericardial cavity; the outer, occasionally united with the sexual orifices, 

 opens upon the external surface of the body. In the analogous, generally un- 

 paired, often contractile organ of snails, guanin has been demonstrated. The organ 

 is capable, in a remarkable manner, not alone of excreting water from the blood, 

 but also of conveying water into the blood. Cephalopods possess sacculated ex- 

 cretory organs, provided with glands and opening into the mantel-cavity lying 

 on the vascular trunks of the gills. 



Insects, spiders, and centipedes have so-called Malpighian vessels, partly as 

 uric-acid forming excretory organs; partly, also, as biliary organs. These vessels 

 are long tubes that open into the commencement of the large intestine. In 

 crabs, the blind tubes of the digestive tract probably have similar functions. In 

 cestodes, the excretory organs are longitudinal tubes; in tape-worms two that 

 extend throughout the entire chain, in the teniag anastomosing at the junction of 

 the segments by means of a large communication. In trematodes (distomum) 

 the branching organ opens at the posterior extremity of the body. Also in most 

 round-worms the excretory organ is formed of tubes, which, united, open at a 

 pore in the abdominal line. Earth-worms possess, almost in all segments of the 

 body in pairs, the so-called nephridia-canals, that is, tubes, often much con- 

 voluted, that commence in the abdominal cavity with an inner, ciliated orifice, 

 and communicate upon the ventral aspect of the body with the external surface . 

 In the sea-urchin, the star-fish, and the medusae, the water- vascular system is, at 

 the same time, the excretory organ. Also in sponges, the canals passing through 

 the body and conveying water may be considered as such. 



Historical. According to Aristotle the urine is derived from the blood passing 

 through the kidneys, and then flows through the ureters into the bladder; the 

 venous blood of the kidneys does not undergo coagulation. He pointed out the 

 relatively large size of the human bladder. Berengar (1521) observed, on injecting 

 water into the renal vessels, that fluid escaped from the papilla?. Massa (1552) 

 discovered lymphatic vessels in the kidneys. Eustachius (died 1580) ligated the 

 ureters and subsequently found the bladder empty. Cusanus (1450) studied 

 the color and the specific gravity of the urine. Rousset (1581) pointed out the 

 muscular nature of the walls of the bladder, in which Sanctorius (1631) was un- 

 able to recognize any special sphincter muscle; while Vesling (1641) had already 

 described the trigone of Lieutaud (1753). The first more important chemical 

 investigations were made by van Helmont in 1644. He demonstrated the solid 

 constituents of the urine, and found among them sodium chlorid. He noted the 

 higher specific gravity of febrile urine, and explained the development of urinary 

 calculi from the solid constituents of the urine. With respect to the discovery 

 of individual urinary constituents, it may be noted that Scheele, in 1776, dis- 

 covered uric acid; Bergmann calcium phosphate; Brand and Kunckel phosphorus; 

 Rouelle, in 1773, urea, which was named by Fourcroy and Vauquelin in 1799; 

 Berzelius lactic acid; Seguin albumin in pathological urine; J. v. Liebig hippuric 

 acid; Heintz and v. Pettenkofer kreatin and kreatinin; Wollaston, in 1810, 

 cystin; Marcet, in 1817, xanthin; Lindbergson magnesium carbonate. The more 

 recent histological, physiological, and chemical investigations are discussed in the 

 text. 



FUNCTIONS OF THE EXTERNAL INTEGUMENT. 

 STRUCTURE OF THE SKIN. 



The external integument, from 2.3 to 2.7 mm. thick, with a specific gravity of 

 1057, is constituted of the cults rcni, corium, cutis, and the overlying epidermis. 



The corium (Fig. 178, I, C) forms upon the entire surface numerous papillae, 

 from o.i to 0.5 mm. high, of which the largest are encountered upon the palmar 

 aspect of the hand and the plantar aspect of the foot, as well as upon the nipple 

 and the glans penis. The majority of the papillae contain loops of capillary 

 blood-vessels (g) , and in circumscribed areas of the skin also so-called tactile 

 corpuscles (Fig. 179, a). The papillae are arranged upon the skin in groups in 

 the small areas bounded by the delicate furrows in the skin that are still macro- 

 scopically visible. On the palmar aspect of the hand and the plantar aspect of 

 the foot they follow the characteristic cutaneous lines. The horny skin consists 



