CHAMBERS'S INFORMATION FOR THE PEOPLE. 



ducts, the exact mode of origin of which is 

 at present obscure. These ducts convey the 

 bile out of the liver ; and after it has become 

 inspissated and mixed with mucus, from small 

 mucous glands in the larger ducts, and from the 

 gall-bladder, it is poured into the duodenum. The 

 liver performs at least three functions : first, the 

 secretion of bile ; second, the formation of fat ; 

 and third, the formation of animal starch or 

 glycogen. The bile is to be regarded chiefly as 

 an excretion rich in hydro-carbons, but during its 

 passage from the economy, it performs certain 

 functions referred to under Digestion. It is highly 

 probable that part of the bile is re-absorbed into 

 the blood, but its ultimate function is unknown. 

 The amount formed daily is about 3^ pounds ; 

 but the quantity is liable to great variation. 



The formation of animal starch by the liver is 

 called \\s glycogenic function. It is supposed that 

 this starch, formed in the cells of the liver, is con- 

 verted by the blood of the hepatic vein into sugar, 

 which is carried to the lungs, where it is decom- 

 posed into carbonic acid and water. This, how- 

 ever, is a point not yet conclusively settled. 



3. The Skin. This organ, continuous at various 



points with the internal 

 mucous surfaces, covers 

 the whole body, and 

 consists of two layers : 

 first, a hard epithelium, 

 composed of cells more 

 or less flattened, called 

 the epidermis ; and 

 second, of the derma, or 

 cutis vera, or true skin, 

 which is formed of con- 

 nective and elastic tis- 

 sue. Underneath the 

 true skin, we find a 

 layer of fat We find 

 in the skin two kinds 

 of glands. The sudorip- 

 arous or sweat glands, 

 consist of a tube, coiled 

 into a ball at the deeper 

 part, and communi- 

 cating with the surface 

 by a spiral duct. The 

 sebaceous glands are 



Fig. 5. Vertical Section of small racemose glands, 

 the Skin of the Sole : which usually open into 



a, cuticle; b, papillary struc- the hair follicles, and 

 ture; c cutis vera or true secrete an oily fluid 

 skin ; a. sweat-gland lyine in /- i > . .-, , 



a cavity on tfa* deep surface lubricating the hairs 



of the skin, and imbedded in and Surface of the skin, 



globules of fat Its duct is -pup r hief pvrrptinn nf 



seen passing to the surface. ' 



Magnified about 30 diameters. Skin IS Sweat, an acid 



watery fluid, which is 



usually carried off from the surface in the form of 

 vapour. The amount varies greatly : from five 

 pounds in the twenty-four hours, to one pound. 

 That the separation of this excretion is important, 

 is proved by the fact, that if the skin be varnished 

 over, so as to prevent exhalation, death may 

 speedily ensue. All the various modifications 

 of epidermis, such as hair, horn, nail, hoof, &c. 

 may also be regarded in the light of excretions, 

 but space compels us merely to allude to this 

 fact. 



4. The Kidneys. The human kidneys are situ- 

 ated in the loins, one on each side of the spine. A 



122 



well-developed healthy kidney weighs about six 

 ounces. When cut open, we find a cavity com- 

 municating with the ureter, the excretory duct 

 of the kidney, and we observe also that the organ 

 consists of two substances, which are named, from 

 their position, the external or cortical, and the 

 internal or medullary substance. The medullary 

 part consists of straight tubules, which divide in 

 two as we pass outwards to the cortical part, while 

 in the latter, the tubes are extremely convoluted. 

 In the cortical part of an injected kidney, there 

 are numerous small round balls of capillaries 

 called Malpighian bodies, after the celebrated 

 anatomist who first observed them. In man they 

 are about the rhrth of an inch in diameter. They 

 consist of a mass of minute capillaries supplied 

 with blood by an afferent vessel, and having also 



Fig. 6. Plan of the Renal Circulation : 

 a, terminal branch of the artery, giving the terminal twig, of, to 

 the Malpighian tuft, m, from which emerges the efferent vessel, 

 ef. Other efferent vessels, e, e, e, are seen proceeding from other 

 tufts, and entering the capillaries surrounding the uriniferous 

 tube, t. From this plexus of capillaries the emulgent vein, ev, 

 springs. 



an efferent vessel to carry the blood away. Each 

 of these little balls is embraced by the dilated end 

 of one of the uriniferous tubes, as seen in the 

 preceding figure. The function of the kidney is 

 to excrete urine, a fluid rich in nitrogenous 

 materials. The urine is an amber-coloured liquid, 

 having a specific gravity of 1020, a slightly acid 

 reaction, a saltish taste, and an odour peculiar to 

 itself. The chief substance in the urine of man is 

 urea, of which about an ounce is excreted daily, 

 while, during the same time, about eight grains of 

 uric acid are separated. In reptiles, however, the 

 amount of urea is small, while uric acid is largely 

 present. These substances are derived from the 

 waste of the nitrogenous tissues, uric acid being 

 formed before urea. In rapid-breathing, warm- 

 blooded animals, uric acid is rapidly oxidised to 

 urea, and consequently only a small amount of the 

 acid appears in the urine ; whereas, in slow- 

 breathing, cold-blooded animals, the oxidation is 

 incomplete. Man being omnivorous, partakes of 

 enough of food rich in carbon to prevent the com- 

 plete oxidation, and therefore a small amount of 

 uric acid is always found in his urine. The kid- 

 neys also separate inorganic matters, such as 

 chlorides, sulphates, phosphates, &c. These are 

 much modified as to amount by the nature of the 

 diet, and the amount of fluid taken with them. 



5. The Intestines. The excretions from the 

 bowel consist not only of the non-nutritious mate- 

 rials of food, bile, mucus, and mineral matters, 



