All these operations paralyze the vaso-motor nerves by which, in health, the blood-vessels of the liver are kept in a state of tonic contraction; hence these vessels dilate when the nerves are cut. From the facts named we also learn the path of the glycogenic influence, which must be from the medulla oblongata into the spinal cord, thence by the filaments of the { 197} sympathetic which accompany the vertebral artery into the lower cervical ganglion; thence through the annulus of Vieussens into the first dorsal ganglion; and thence through the prevertebral cord of the sympathetic, and branches not precisely determined, to the hepatic blood-vessels as shown by the dotted line in Fig. 3.
[Illustration: FIG. 3. Diagram showing the course of the vaso-motor nerves of the liver, according to Cyon and Aladoff. These nerves are indicated by the dotted line which accompanies them: _a_, vaso-motor centre; _b_, trunk of the vagus; _c_, passage of the hepatic vaso-motor nerves from the cord along the vertebral artery; _d_, fibres going on each side of the subclavian artery and forming the annulus of Vieussens; _e_, first dorsal ganglion; _f_, ganglionated cord of the sympathetic; _g_, the spinal cord; _h_, the splanchnic nerves; _i_, coeliac ganglion, from which vaso-motor nerves pass to the hepatic and intestinal vessels; _k_, the lungs, to which fibres of the vagus are seen distributed; _l_, the liver; _m_, the intestine; _n_, the arch of the aorta.]
I say, by branches of the sympathetic not precisely determined, because our power to produce artificial diabetes fails below the first thoracic ganglion; for section of the sympathetic between the tenth and twelfth ribs, and of the splanchnics, is not followed by glycosuria, although the vaso-motor nerves to the liver are known to pass through them.
According to Eckhard,[3] the phenomena of artificial glycosuria are irritative and not paralytic. This view he believes sustained by his own experiments, according to which if the splanchnics, through which { 198} the vaso-motor nerves of the liver pass, are cut prior to the diabetic puncture, not only does this operation fail to produce glycosuria, but it even renders ineffectual the puncture itself as well as the section higher up. But Cyon and Aladoff remind us that it is not mere dilatation of the hepatic vessels, but increased velocity in the movement of the blood, which deranges the sugar-assimilating function and causes glucose to appear in the urine. The vaso-motor nerves of the intestinal blood-vessels also pass through the lower part of the sympathetic and the splanchnics, and section of the latter must cause these blood-vessels to dilate. Now, in rabbits, in which this experiment is usually performed, the digestive canal is very long, and the blood-vessels so capacious that when dilated they hold as much blood as all the rest of the vascular system together, so that when the lower sympathetic and splanchnics are cut, so much blood goes into the intestines that the increased velocity required in the blood-vessels of the liver to produce glycosuria is impossible. But if the vessels of the liver be first dilated by puncturing the floor of the fourth ventricle, section of the sympathetic or of the splanchnics may then be made without arresting the formation of sugar; whence it would appear that the glycogenic influence may still pass through the lower sympathetic and splanchnics.