Of hippuric acid, which exists in small quantity in human urine, but in much larger amount in the urine of herbivorous animals, Dr. Garrod states (loc. cit.) that he thinks he has detected traces in the blood.
There can be no reasonable doubt that kreatine and kreatinine are normal elements of healthy blood, since they are constituents of the "juice of flesh," which seems to be the result of the disinteg,ration of the muscular tissue, and must be taken into the circulating current to be conveyed from the muscles into the urine, where we again meet with these substances.
In like manner it is probable that lactic acid is normally present in the blood in very mi nute proportion ; for it abounds in the juice of flesh, and must be taken into the current of the circulation, in order to be eliminated from the body. In the healthy state it seems to be eliminated through the respiratory organs as fast as it is generated; being con verted by oxidation into carbonic acid and water. It was formerly supposed to be a normal constituent of the urine; but it has been clearly proved by Liebig not to have a real existence there. Even when lactate of potash has been introduced by the stomach, the potash is thrown out by the kidnejs in combination with other acids, the lactic acid not being eliminated in the urine, but passed off through the lungs. In certain dis eased states of the system, however, lactic acid unquestionably presents itself in the gas tric, urinary, and cutaneous secretions ; and as it has been shown to be one of the results of the disintegration of the muscular tissue, its pre-existence in the blood cannot be reason ably doubted.
The less definite nature of the constituents of bile prevents them from being as certainly re cognised in the blood as those of urine have been ; nevertheless, the evidence of their pre existence in the circulating fluids is sufficiently clear. Thus cholesterine may be obtained from the serum of the blood by an analytical process of no great complexity ; and its pre sence there is also manifested by its occasional deposit, as a result of diseased action, in other parts of the body, especially in the fluids of local dropsies, as hydrocele, ovarian dropsy, &c. Again, the colouring matter of the bile seems to be nearly identical with certain normal elements of the blood, since the hue exhibited by a departing ecchymosis is identical with the characteristic colour of bile. In cases of jaundice, the presence of
the colouring matter in the blood is often made evident, not nierely by the communica tion of its peculiar hue to the several tissues and secretions of the body, but also by the tint visible in the serum of the blood itself. It would seem probable, however, that in many of these cases there has been an actual re-absorption of the biliary matter subse quently to its elimination by the liver, as a consequence of obstruction to its exit by the gall duct. But in the most severe and ra pidly fatal cases ofjaundice, as pointed out by Dr. Alison*, the secreting process has never taken place, and the colouring matter must then have been generated in the blood itself. Neither cholesterine, nor the colouring matter of bile, seem to exert the poisonous influence on the nervous system which is manifested in the cases alluded to ; and it is probable that this must proceed from the accumulation of the peculiar organic constituent of the bile. As the precise nature of this, however, is still a matter of discussion amongst chetnists, we cannot be surprised that it has not yet been obtained by analysis from the blood.
The proof that the constituents of the milk pre-exist in the blood, is rather inferential than direct. That the caseine (although so like the albumen of the blood that we might imagine it to be a mere modification of it, effected in the act of secretion) is, in reality, specially prepared in the circulating current, would appear from the fact that, during preg nancy, a substance, kiestein, having a close relation to it, is eliminated by the urine, and that this substance disappears from the urine within a few days after parturition, the mam mary secretion being then fairly established. Perhaps, however, the most remarkable evi dence to the same effect is afforded by cases of metastasis of the mammary secretion, of which an account will be presently given ; and on the same kind of evidence rests the proof of the pre-existence of the other cha racteristic elements of the mammary secretion in the blood.