urine, as we have already stated, contains no free acid, its acid. reaction to test paper being dependent entirely on the presence of biphosphate of soda. Lactic acid nevertheless has been so frequently found in nearly fresh urine as to lead some eminent chemists (Berzelius, Lehmann) to regard it as a natural constituent of the excretion. It has been subsequently found, however, that urine, though entirely free from lactic when first passed, may frequently present traces of this substance after some hours' exposure to the air. The lactic acid is undoubtedly formed, in these cases, by the decomposition of some animal substance contained in the urine. Its production in this way, though not constant, seems to be sufficiently frequent to be regarded as a normal process.

In consequence of the presence of this acid, the urates are partially decomposed; and a crystalline deposit of free uric acid slowly takes place, in the same manner as if a little nitric or muriatic acid had been artificially mixed with the urine. It is for this reason that urine which is abundant in the urates frequently shows a deposit of crystallized uric acid some hours after it has been passed, though it may have been perfectly free from deposit at the time of its emission.

During the period of the "acid fermentation," there is reason to believe that oxalic acid is also sometimes produced in a similar manner with the lactic. It is very certain that the deposit of oxalate of lime, far from being a dangerous or even morbid symptom, as it was at one time regarded, is frequently present in perfectly normal urine after a day or two of exposure to the atmosphere. We have often observed it, under these circumstances, when no morbid symptom could be detected in connection with either the kidneys or any other bodily organ. Now, whenever oxalic acid is formed in the urine, it must necessarily be deposited under the form of oxalate of lime; since this salt is entirely insoluble both in water and in the urine, even when heated to the boiling point. It is difficult to understand, therefore, when oxalate of lime is found as a deposit in the urine, how it can previously have been held in solution. Its oxalic acid is in all probability gradually formed, as we have said, in the urine itself; uniting, as fast as it is produced, with the lime previously in solution, and thus appearing as a crystalline deposit of oxalate of lime. It is much more probable that this is the true explanation, since, in the cases to which we allude, the crystals of oxalate of lime grow, as it were, in the cloud of mucus which collects at the bottom of the vessel, while the

supernatant fluid remains clear. These crystals are of minute size,

Fig. 116.

transparent, and colorless, and have the form of regular octohedra, or double quadrangular pyramids, united base to base. (Fig. 116.) They make their appearance usually about the commencement of the second day, the urine at the same time continuing clear and retaining its acid reaction. This deposit is of frequent occurrence when no substance containing oxalic acid or oxalates has been taken with the food. At the end of some days the changes above described come to an end, and are succeeded by a different process known as the alkaline fermentation. This consists essentially in the decomposition or metamorphosis of the urea into carbonate of ammonia. As the alteration of the mucus advances, it loses the power of producing lactic and oxalic acids, and becomes a ferment capable of acting by catalysis upon the urea, and of exciting its decomposition as above. We have already mentioned that urea may be converted into carbonate of ammonia by prolonged boiling or by contact with decomposing animal substances. In this conversion, the urea unites with the elements of two equivalents of water; and consequently it is not susceptible of the transformation when in a dry state, but only when in solution or supplied with a sufficient quantity of moisture. The presence of mucus, in a state of incipient decomposition, is also necessary, to act the part of a catalytic body. Consequently if the urine when first discharged be passed through a succession of close filters, so as to separate and retain its mucus, it may be afterward kept, for an almost indefinite time, without alteration. But under ordinary circumstances, the mucus, as soon as its putrefaction has commenced, excites the decomposition of the urea, and carbonate of ammonia begins to be developed.

OXALATE OF LIME; deposited from healthy urine, during the acid fermentation.

The first portions of the ammoniacal salt thus produced begin to neutralize the biphosphate of soda, so that the acid reaction of the urine diminishes in intensity. This reaction gradually becomes

weaker and weaker, as the fermentation proceeds, until it at last disappears altogether, and the urine becomes neutral. The production of carbonate of ammonia still continuing, the reaction of the fluid then becomes alkaline, and its alkalescence grows more strongly pronounced with the constant accumulation of the ammoniacal salt.

The rapidity with which this alteration proceeds depends on the character of the urine, the quantity and quality of the mucus which it contains, and the elevation of the surrounding temperature. The urine passed early in the forenoon, which is often neutral at the time of its discharge, will of course become alkaline more readily than that which has at first a strongly acid reaction. In the summer, urine will become alkaline, if freely exposed, on the third, fourth, or fifth day; while in the winter, a specimen kept in a cool place may still be neutral at the end of fifteen days. In cases of paralysis of the bladder, on the other hand, accompanied with cystitis, where the mucus is increased in quantity and altered in quality, and the urine retained in the bladder for ten or twelve hours at the temperature of the body, the change may go on much more rapidly, so that the urine may be distinctly alkaline and ammoniacal at the time of its discharge. In these cases, however, it is really acid when first secreted by the kidneys, and becomes alkaline while retained in the interior of the bladder.

The first effect of the alkaline condition of the urine, thus produced, is the precipitation of the earthy phosphates. These salts, being insoluble in neutral and alkaline fluids, begin to precipitate as soon as the natural acid reaction of the urine has fairly disappeared, and thus produce in the fluid a whitish turbidity. This precipitate slowly settles upon the sides and bottom of the vessel, or is partly entangled with certain animal matters which rise to the surface and form a thin, opaline scum upon the urine. There are no crystals to be seen at this time, but the deposit is entirely amorphous and granular in character.

The next change consists in the production of two new double salts by the action of carbonate of ammonia on the phosphates of soda and magnesia. One of these is the "triple phosphate," phos phate of magnesia and ammonia (2MgO,NH,O,PO,+2HO). The other is the phosphate of soda and ammonia (NaO,NH,O,HO,PO,+ 8HO). The phosphate of magnesia and ammonia is formed from the phosphate of magnesia in the urine (3MgO,PO,+7HO) by the replacement of one equivalent of magnesia by one of ammonia. The crystals of this salt are very elegant and characteristic. They

show themselves throughout all parts of the mixture; growing gradually in the mucus at the bottom, adhering to the sides of the

Fig. 117.

PHOSPHATE OF MAGNESIA AND AMMONIA;

glass, and scattered abundantly over the film which collects, as we have mentioned, upon the surface. By their refractive power, they give to this film a peculiar glistening or iridescent appearance, which is nearly always visible at the end of six or seven days. The crystals are perfectly colorless and transparent, and have the form of triangular prisms, generally with bevelled extremities. (Fig. 117.) Frequently, also,

deposited from healthy urine, during alkaline fermen- their edges and angles are

tation.

replaced by secondary facets. They are insoluble in alkalies, but are easily dissolved by acids, even in a very dilute form. At first they are of minute size, but gradually increase, so that after seven or eight days they may become visible to the naked eye.

The phosphate of soda and ammonia is formed, in a similar manner to the above, by the union of ammonia with the phosphate of soda previously existing in the urine. Its crystals resemble very much those just described, except that their prisms are of a quadrangular form, or some figure derived from it. They are intermingled with the preceding in the putrefying urine, and are affected in the same way by chemical reagents.

As the putrefaction of the urine continues, the carbonate of ammonia which is produced, after saturating all the other ingredients with which it is capable of entering into combination, begins to be given off in a free form. The urine then acquires a strong ammoniacal odor; and a piece of moistened test paper, held a little above its surface, will have its color immediately turned by the alkaline gas escaping from the fluid. This is the source of the ammoniacal vapor which is so freely given off from stables and from dung heaps, or wherever urine is allowed to remain and decompose. This process continues until all the urea has been destroyed, and until the products of its decomposition have either united with other substances, or have finally escaped in a gaseous form.

SECTION II.

NERVOUS SYSTEM.

CHAPTER I.

GENERAL STRUCTURE AND FUNCTIONS OF THE

NERVOUS SYSTEM

IN entering upon the study of the nervous system, we commence the examination of an entirely different order of phenomena from those which have thus far engaged our attention. Hitherto we have studied the physical and chemical actions taking place in the body and constituting together the process of nutrition. We have seen how the lungs absorb and exhale different gases; how the stomach dissolves the food introduced into it, and how the tissues produce and destroy different substances by virtue of the varied transformations which take place in their interior. In all these instances, we have found each organ and each tissue possessing certain properties and performing certain functions, of a physical or chemical nature, which belong exclusively to it, and are characteristic of its action.

The functions of the nervous system, however, are neither phy. sical nor chemical in their nature. They do not correspond, in their mode of operation, with any known phenomena belonging to these two orders. The nervous system, on the contrary, acts only upon other organs, in some unexplained manner, so as to excite or modify the functions peculiar to them. It is not therefore an apparatus which acts for itself, but is intended entirely for the purpose of influencing, in an indirect manner, the action of other organs. Its object is to connect and associate the functions of different parts of the body, and to cause them to act in harmony with each other.