that present in arterial blood, and the amount present is at its minimum in the venous blood of active muscle'. Sugar is therefore consumed by living muscle, especially during activity. The amount of sugar present in hepatic venous blood is constantly slightly above that present in portal venous blood. Sugar is therefore produced by the liver. We have now no difficulty in following the sugar-cycle in an active well-fed animal. Absorbed sugar enters the blood, is to a great extent stored as glycogen in the liver, is consumed by living muscle, is discharged as CO, and as H2O. The part played by glycogen is that of a temporary carbohydrate reserve.

In a starving animal, receiving no fresh supply of sugar, sugar is nevertheless a necessity; the glycogen store soon disappears; yet the animal lives on for a time, having sugar in its blood, and using up sugar in its muscles. It is probable that this sugar is made by the tissues from the tissues (by the liver and possibly by muscle, from proteid and possibly from fat). From this point of view the fundamental chemical event is the formation and consumption of sugar; the formation of glycogen is a first incident in this event, occurring whenever the carbohydrate supply is plentiful, and failing to occur when the food-supply is suspended.

It is instructive to observe that this reserve carbohydrate function is represented throughout the animal and vegetable kingdoms; the liver makes and puts by glycogen at one time, transforms and discharges glycogen at another time; a potato makes and puts by starch at one time, transforms and discharges starch at another time. The alternate phases are of very dif ferent duration in the different cases; the carbohydrate reserve remains for a few hours or days in the liver of a warm-blooded animal, for many days or weeks in the liver of a cold-blooded animal, for several months in the tuber of a potato, or in the liver of a hibernating animal.

Alterations of the liver structure in consequence of digestion are obvious enough both to the naked eye and microscopically, but the interpretation of their significance in all their visible details is rendered uncertain, mainly because we have to deal with two possible altering agents--with an outgoing current of secretion as in other glands, and with an incoming current of

Chauveau estimated, e.g., that 175 grms. of blood traversed 1 kilo of resting muscle in 1 minute, losing 0-036 grm. sugar, whereas through active muscle the quantities per minute were 850 grms. and 0-140 grm.

food-products. It is to the last-named factor that the more obvious changes are due. The liver of a rabbit dieted in preparation for glycogen extraction is large, pale, and twice as heavy as the liver of a similar rabbit not so prepared; and whereas the liver of the fed rabbit may yield over 10 grammes of glycogen, that of the other rabbit would yield less than 1 gramme. A microscopic section of a dog's liver during the fasting state

FASTING.

AFTER FOOD.

FIG. 89.

Liver-cells of dog after a thirty-six shows uniformly granular

hours' fast, and fourteen hours after a full meal-in the latter case swollen with glycogen. (Heidenhain.)

shrunken cells with obscured

nuclei; taken twelve or fourteen hours after a copious meal, the

cells are distended with coarse clumps, which give a red-brown colour on irrigation with iodine, and which, when they dissolve, leave the cells with distinct boundaries and distinct nuclei in a ragged intra-cellular network of protoplasm. The appearance is evidently due to a deposition of and distension by glycogen during the post-prandial period.

Changes in the main similar to the above and attributed to the glycogenic process have also been made out upon frogs,

Rabbit's liver twelve hours after a small meal. Iron deposit round portal veins, glycogen deposit round hepatic veins; (at an earlier period the glycogen pervades the whole of the liver). From micro-photographs of a liver treated with ferrocyanide of potassium and hydrochloric acid to show the iron, with iodine to show the glycogen. Hepatic vessels in the iron section, and portal vessels in the glycogen section, are very indistinct. (Delépine.)

in which the liver is of distinctly tubular type. According to Langley, the liver of a winter frog exhibits a distinction into

zones similar to those of the salivary glands; in this case the outer hyaline zone is loaded with glycogen, the inner granular zone with material presumably proteid, but which, seeing that the liver yields no definite ferment, cannot be termed 'zymogen.'

Another material that is apt to be deposited in the liver-cells, both of mammalia and of frogs, is fat, which forms droplets more or less abundantly in accordance with the state of nutrition (e.g. the fatty livers of crammed geese or of overfed and ill-nourished persons), but in no causal relation with the particular phase of digestion.

Finally, we have to recognise that the hæmolytic function of the liver may be attended with visible changes. According to Delépine, a granular deposit giving the ferric reaction exists in the liver-cells slightly during fasting, not at all immediately after a meal, most abundantly eight to twelve hours later; and whereas the glycogen deposit is most abundant around hepatic veins, the iron deposit is thickest round the portal veins.

MECHANISM OF ITS

224 THE URINE-quantity-reaction-specific gravity-composition. 226

SECRETION. Theories of Bowman-of Ludwig-of Heidenhain-Water through glomeruli, urea through tubules '-Observations of Heidenhain-of Nüssbaum - Effects of blood-pressure-of state of blood-of nerve-action-Use of the renal oncometer-The renal nerves. 231 UREA-Made by the tissues, discharged into the blood, separated by the kidneys'-'formed in chief part from food proteid, in small part from body proteid '-Proofs of these statements.

233 Detection, preparation, synthesis, and estimation of urea-Hypobromite method-Distillation method.

233 Uric acid-Hippuric acid-Creatinin-Salts-Chlorides, phosphates and sulphates-Pigments-Urobilin.

238 Abnormal constituents-Blood, albumin, and sugar-Estimation of sugar by copper, by fermentation, by the polarimeter.

240 Micturition-vesical pressure.

241 Excretory action of the liver.

243 Diabetes.

244 *Obscure facts relating to glands and nutrition; 'internal secretion pancreatic diabetes, thyroid disease, partial excision of kidney.

245 Excretory action of the skin-Sensible and insensible perspiration-Sudomotor nerves-Atropin and Pilocarpin-Pilomotor nerves-Contraction of pigment cells.

The relation of excretion to digestion.-Respiration, digestion, circulation, and excretion are the agencies through which the blood is maintained of such quality and composition that it serves throughout life as the vehicle of nourishment and of purification to the whole body. Respiration is essentially a process of exchange, formed by a double current of gases -of incoming oxygen, and of outgoing carbon dioxide. Digestion and excretion similarly constitute a process of exchange, in which, however, the two currents run in separate channels; carbon and nitrogen entering the body together by intestinal absorption, but nitrogen leaving the body by renal excretion as urea, and carbon by respiratory excretion as CO2. And whereas in the absorption of oxygen the gas exists ready in the air, and enters the body unaltered, carbon and nitrogen are con

veyed into the body in certain combinations or proximate principles (carbohydrates, fats, and proteids) which form part of our ordinary articles of diet, and require to be prepared for absorption by digestion in the alimentary canal before they can actually enter the body. Thus respiration is simple absorption and excretion of gases, digestion is the preparation for absorption, and actual absorption of proteids, of carbohydrates, and of fats. Indigestible substances simply pass through the alimentary canal without actually entering the body, and are got rid of by defæcation, in company with an insignificant amount of matter actually excreted by the intestinal canal. The principal excretions, properly so called, are CO, by respiration, and urea by renal action. We have already considered the former, we are about to consider the latter.

Pyramid of Malpighi
Column of Bertini

Renal artery

Pelvis

Papillary zone of medulla

Boundary zone of medulla
Cortex
Ureter

FIG. 91.-DIAGRAMMATIC SECTION THROUGH THE KIDNEY.

The branches of the renal artery pass along the columns of Bertini and form a series of arterial arches between cortex and medulla; from these arches spring the interlobular arteries and the vasa recta. (See fig. 93.)

The kidney.—A human kidney sliced longitudinally exhibits an outer zone the cortex-surrounding an inner portion-the medulla. The medulla is composed of twelve to fifteen pyramidal masses; its outer portion in contiguity with the cortex is called the boundary zone; its inner portion is termed the papillary zone, each pyramid terminating as a papilla which protrudes into the pelvis of the kidney; the pelvis is the common central cavity and the commencement of the ureter. The structural characters of the several portions of the kidney are determined by peculiarities in the anatomical disposition of (1) the blood-vessels, (2) the urinary tubules. Microscopically, the cortex is characterised by convoluted tubules in such mass as to be termed 'the labyrinth'; it also contains bundles of straight tubules forming the medullary rays. The boundary zone of the medulla is characterised by