Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/740

682 NUTRITION this there is direct experimental proof in the facts that if sugar be slowly injected into the jugular vein, no escape of it by the kidneys takes place, because the organism can make use of a small excess of sugar; but if it be injected quickly diabetes ensues from the too rapid accumulation of it in the blood. Accepting this, ve can readily see that diabetes might be caused (1) by the failure of the liver to pick out the sugar in the portal vein in order to store it, (2) by an unusually large escape of glycogen from the liver in the form of sugar, (3) by the failure of the distant organs whose func tion it is to burn off the daily quantum of sugar. It is not improbable that there may be different sorts of diabetes arising from different causes. Thus it has been said that the diabetes of curare-poisoning appears even when the liver has previously been emptied of glycogen by starving the animal. When we reflect that muscles are the organs especially affected by curare, the fact (if it be a fact) just mentioned at once suggests that the sugar which appears in the urine during curare-poisoning is due to some defect in the muscular tissue, in consequence of which the glycogen normally used up by it fails to be used and passes back into the blood. Further, the failure of glycogen-using organs may be due to no disqualification of theirs for the work of utilizing normal glycogen, but to some abnormal constitution of the glycogen (or the sugar) offered to them. Again, the diabetes caused by copious injection of dilute saline solutions into the blood-vessels may be due to the large destruction of blood-corpuscles which follows such in jections, and the consequent production of a large quantity of the starch-converting substance which is known to arise in the disin tegration of blood-cells. But the common form of artificial diabetes "puncture diabetes, " as it is sometimes called the diabetes of anatomical nervous lesions, is undoubtedly due to defects in the hepatic apparatus brought about by the injury. Thus the puncture of the medulla oblongata is of no avail to cause diabetes if the liver has first been deprived of its glycogen by starving the animal experimented upon. It is, however, far from necessary to suppose that the puncture diabetes is due to any incapacity of the liver- cells for their usual metabolic changes ; this may be the cause of some kinds of pathological diabetes, but it is almost certainly not the cause of the traumatic variety, and for the following reason. If sugar be freely administered to an animal in which the diabetic nervous lesion has been established, although it is impossible to load the liver with glycogen as, in such conditions, it would become loaded during health, yet a certain amount of glycogen does become stored in the liver. The liver seems still to possess the power of converting the sugar of food into a store of starch, but not the ful ness of opportunity which it has in health. The conditions which deprive the liver of the opportunity of storing glycogen are to be sought in an altered vaso-motor mechan ism. The circumstance that the "diabetic centre " in the medulla coincides with part of the general vaso-motor centre at once leads to the vaso-motor theory of diabetes ; but it may be admitted that this consideration is almost the strongest support of the theory, How a vaso-motor paralysis brings about diabetes we have as yet no clear picture. Perhaps it is by permitting such a rapid current of blood through the vessels that there is not time enough for the usual transmutations. If we may assume this to be the method by which diabetes is produced, then the vaso-motor theory receives much support from the following consideration. Inasmuch as diabetes is caused by a local (hepatic) dilatation of blood-vessels leading to a superabundant torrent of blood, if means be taken to provide other channels for the blood and so relieve the superabund ance, diabetes should be prevented. This may be done by dividing the splanchnic nerves and dilating the enormous vascular tracts of the abdominal organs. In these circumstances the diabetic puncture makes but a trivial change in the circulation of the liver compared with that which it makes when the blood -pressure is normal; and, as might have been foretold from the vaso-motor theory, no diabetes follows. If diabetes from nervous lesions is a vaso-motor phenomenon, other forms of artificial diabetes are not so. Arsenic and antimony seem to destroy the power of the hepatic cells to manufacture glycogen ; sugar ingested with the food passes, in consequence of the failure, directly into the blood and out of it into the urine. Ligature of the bile-duct also rapidly leads to profound disorganization of the hepatic tissues, in conse quence of which imported sugar passes the portal system unheeded and escapes by the kidneys. Metabolism of the Blood. We have hitherto regarded the blood as subordinately related to the great processes of the body, as playing the mechanical part of a carrier, and as if it had no direct interest in the metabolic opera tions themselves ; the reader, however, need scarcely be re minded that, but for the respiratory exchanges between the blood on the one hand and the anatomical elements of the tissues on the other, all metabolic processes would be im possible. In the article RESPIRATION, and in treating the particular section " the respiration of the tissues," the influence of those exchanges on metabolism will be care fully considered. The older physiologists regarded the blood as the very seat of the chemical changes of the body, a view which is almost entirely opposed to that which we now hold, and which is disproved by many facts. We have little or no evidence that the blood exhibits transformations of matter such as we have been discussing in the case of the tissues generally. There are few such transformations known to us in which carbonic acid is not one of the most abundant end-products of the change, and therefore, when we find that blood removed from contact with the tissues and freed from carbon dioxide produces very little of this body, although an abundance of oxygen be supplied to it, we have the right to suspect strongly that it is due to the absence of any active metabolism. It is not certain that blood (or lymph), without the intervention of some other tissue, has even the power of converting non-coagulating albumins fresh from the digest ive organs into the characteristic fibrin -formers. It is true that when lactate or caprate of soda is injected into the blood of an animal it is readily oxidized ; but this may easily be due to the operation of some organ through Avhich the blood passes, and which finds the bodies referred to suitable to its own metabolic processes ; and this view of the case is further strengthened when we find that formiate of sodium, Avhich also is a readily oxidized body, passes through the same ordeal unchanged. Nevertheless, the blood is to a certain extent an active living tissue. The white corpuscles, at least, are definite living structures whose life-history, could we but decipher it, would be found to be a history of constant metabolic processes. Towards these elements of the blood the fluid plasma must behave as towards any other tissue-element, supplying them with raw material, and receiving from them the end-products of their internal chemical decom position ; and therefore the blood, as a whole, must be classed among the metabolic tissues. The production of the anatomical elements of the blood does not concern us here ; it may suffice to say that the white cells are added to the blood or lymph from certain structures of which the spleen, thymus body, lymphatic glands, and mucosa of the alimentary canal are the chief, and that probably the red corpuscles are developed from the white ones. It may also be pointed out that there are certain parts of the body e.g., the spleen, liver, marrow of bones, &c. where there is evidence to show that red corpuscles undergo rapid and wholesale destruction, either for the production of some secretion, or possibly because the corpuscles have become old and unfit for work, and their invaluable con stituent, iron, is wanted for younger and more active corpuscles. IV. PROCESSES OF EXCRETION, The blood, as has been more than once said, is subject to continual additions and subtractions on the part of the tissues. The subtractions effected by the tissues are made good in part by the importation of fresh material of food from the alimentary canal. The analogous counterbalanc ing operations which serve to check the accumulation of used-up tissue-substance in the blood take place in certain organs called excretory. In a strict sense, all organs which cast out material from the body are excretory ; the digestive glands, for example, pour their secretions into the aliment ary canal, i.e., outside the strict limits of the body. But so much of their constituents are reabsorbed before the alimentary canal is traversed that they may, for practical purposes, be regarded as never having left the body, Even the constituents of tears, when these do not fall over the