260 A MANUAL OF VETERINARY PHYSIOLOGY 



is never converted back into sugar, but is built up into fat and 

 protein. Further, he denies the existence during life of a liver 

 ferment capable of converting glycogen into glucose. He showed 

 that egg-albumin was capable of being so treated that it yielded 

 a reducing sugar, which was obtained from the carbohydrate group 

 of the protein molecule, and serum globulin behaves similarly ; 

 mucin and nucleo-albumin are also now known to contain the 

 same substance. The importance of this discovery was consider- 

 able, as the production of sugar from protein, though suspected, 

 had not been proved. The diet of the omnivora and herbivora 

 contains more carbohydrate than can be accounted for in the 

 muscles and liver as glycogen, and it is certain that all over and 

 above that required for the purposes of sugar must be converted 

 into fat, and some of it incorporated with the protein tissues. 

 Nevertheless the Bernard doctrine explains why the percentage of 

 sugar in the circulating blood is constant — viz., by the gradual 

 doling out of glycogen as sugar from the liver under the influence 

 of a ferment in that gland, and physiologists have, with but few 

 exceptions, accepted his teachings. 



What we have now to consider is the manner in which this 

 regulation is effected. The first to throw some light on the process 

 was Bernard, who showed that if the floor of the fourth ventricle 

 be punctured temporary diabetes results, sugar appearing in 

 the urine, while the liver uses up its glycogen. The spot in the 

 medulla is known as the diabetic centre, and the puncture acts, 

 not by destroying, but by stimulating its activity. This is 

 proved by the fact that if the animal be starved before the 

 puncture is made no sugar appears in the urine, as no glycogen 

 exists in the liver. The nerves passing to this centre — the 

 afferent nerves — are contained in most of the sensory nerves of 

 the body ; if these are stimulated they act reflexly on the diabetic 

 centre, and sugar in the urine results. The same occurs on 

 stimulating the cerebral end of the divided vagus. The impulses 

 from the diabetic centre to the liver pass down the spinal cord, 

 and emerge in the anterior thoracic region with the inferior roots 

 of the spinal nerves, and are connected with the inferior cervical 

 and thoracic ganglia by the rami communicantes ; from these 

 ganglia the impulses pass by means of the splanchnic nerve to the 

 liver. Whether the splanchnic contains fibres which secrete the 

 ferment is unknown, but stimulation of the inferior cervical 

 and thoracic ganglia produces glycosuria provided the splanchnic 

 remains uncut. At one time it was considered that these results 

 were due to vasomotor effect, and that vascular dilatation, 

 rather than secretory activity, was brought about by stimulating 

 the splanchnic, but it is now known that glycosuria is produced 

 when the central end of the depressor nerve is stimulated, and 



