It is evident that an agency involving any part of this tract in such a way as to paralyze the vaso-motor nerves of the liver is capable of producing glycosuria. Such cause may operate upon the central ganglia whence the nerves emanate, as the vicinity of the oblongata and upper parts of the spinal cord or the coeliac ganglion and its branches, including those to the pancreas. Or the irritation may be peripheral and its effects reflex. We have seen that irritation of the central end of the cut vagus will produce glycosuria. Any irritation, therefore, involving the peripheral distribution of this nerve may produce it. Hence embarrassed respiration, whether due to disease of the respiratory passages, strangulation, or inhalation of irrespirable gases and anæsthetics, produces glycosuria in dogs and rabbits; and this symptom has been known to attend these conditions in the human subject. So, too, glycosuria may be produced by such substances as woorara, strychnia, morphia, and phosphoric acid, introduced into the blood and irritating the terminal filaments of the pneumogastrics, or it may be brought about secondarily through the embarrassed respiration these drugs produce. Such peripheral irritation may reside also in the stomach, intestines, liver, or any organ to which the pneumogastric is distributed.
It is not unlikely that irritation of the extremities of sensory nerves other than the pneumogastric may become the cause of reflex glycosuria. Even puncture of the floor of the fourth ventricle itself may be reflex in its operation, the roots of the pneumogastric being thus irritated. The effect of the irritation conveyed to the glycogenic centre is to inhibit the usual tonic influence of the vaso-motor nerve upon the vessel walls. Among the experimental irritations, in addition to puncture of the floor of the fourth ventricle, which produce glycosuria by reflex action, are injuries of the cerebral lobes and cerebellum, optic thalami, cerebral peduncles, pons varolii, middle cerebellar peduncles, and even of the sciatic nerve and brachial plexus; whence it may be inferred that pathological irritation in the same situations may result in a glycosuria, which is temporary or permanent according as the irritation is temporary or permanent.
Finally, there is no reason why an inhibitory reflex action should not originate in the sympathetic itself. When we remember that this nerve is both sensory and motor in function, and that the inhibitory influence to which the heart's action is subject is accomplished through the sympathetic as a sensory nerve and the pneumogastric as a motor, there is no reason why similar results may not be brought about by the sympathetic alone. This being the case, we need not ascribe glycogenic phenomena to irritation in Eckhard's sense—that is, to a direct stimulant action of the irritant upon the vaso-motor nerves of the liver—but may suppose a sensory influence to ascend one set of sympathetic filaments and an inhibitory influence to descend through another.
Dr. Pavy has recently put forward some chemical theories which explain the action of the hyperæmia in producing glycosuria, but they do not account for the hyperæmia itself. In healthy digestion the carbohydrates (starch and sugar) are converted, not into glucose, but into maltose, C12H22O11, dextrin being intermediate in composition. Maltose is absorbed and assimilated, converted into glycogen. So, too, when glucose is ingested as such, it is converted by the glucose ferment into maltose in the stomach and intestines. For the proper production of maltose and its assimilation a good venous blood, producing a maltose-forming ferment, is necessary. In diabetes, in consequence of the dilatation of the arteries of the chylopoëtic viscera, the blood enters the liver too little deoxygenated, and a glucose-forming ferment is produced. The glucose thus formed is not assimilable, but passes off into the circulation and the urine.
MORBID ANATOMY.—Such are some of the facts bearing upon the pathology of diabetes mellitus. Throwing out the milder type of cases, in which glycosuria is the result of an over-ingestion of saccharine and sugar-producing food—and these can scarcely be called instances of essential diabetes—it is evident that glycosuria may be produced in a variety of ways operating through the nervous system; and accordingly we may infer that there is scarcely an organ in close relation with the sympathetic system derangement of which is not capable of producing it. Among these we would naturally expect to find conspicuous alterations in the nervous centres, and yet I have never found changes in these centres after death. At the same time, others have noted meningitis, tubercular and traumatic, apoplectic effusions, and tumors of the brain, especially in the neighborhood of the medulla oblongata. The alterations in the nerve-centres described by Dickinson as the essential morbid anatomy of diabetes I have looked for in vain. These changes are described as a cribriform or porous condition of the white nervous matter, said to be visible to the naked eye. The spaces thus produced are partially occupied by dilated blood-vessels, which, in turn, are surrounded by dilated perivascular sheaths and broken-down nervous matter, into which extravasations of blood have taken place, as evidenced by the presence of pigment-granules. The changes are found in the white matter of the convolutions of the brain, but fewer and larger in the central portions. The corpora striata, optic thalami, pons, medulla, and cerebellum are favorite seats for the largest and most striking holes. In rapidly-fatal cases the cavities are sometimes filled with a translucent, gelatinous substance, containing, besides vascular elements, the globular products of nervous disintegration. In the more chronic forms of the disease, as it occurs in elderly persons, the excavations are usually empty, although the elements of nervous decay are still to be found fringing the margins or collected as an irregular sheath upon the dilated or shrunken artery. There are changes in the cord similar to those in the brain, but less decided. But the most striking alteration in the cord, according to Dickinson, although not always present, is dilatation of the central canal, which in the dorsal and lumbar regions is sometimes expanded to many times its normal diameter, and forms a conspicuous object immediately after the cord is divided.
These alterations have eluded the vigilance of other pathologists who have sought for them in well-determined cases of diabetes mellitus, while they have been found, on the other hand, in the nervous centres when no diabetes was present. In the recent discussion on diabetes at the Pathological Society of London, Douglas Powell7 seemed to be the only one who was convinced that most of Dickinson's specimens were examples of positive lesions.
7 London Lancet, May 5, 1883, p. 776.
A hyaloid thickening of the blood-vessels of the brain has been noted by Stephen Mackenzie8 and Seymour Taylor9 in some cases, and miliary aneurisms of the retina in one.
8 Discussion on Diabetes, Path. Soc. of London, London Lancet, April 7, 1883, p. 593.
9 Ibid., Lancet, May 5, 1883, p. 774.