THE NERVOUS SYSTEM 259 



root by means of strong bundles of fibers; (b) peripheral, with 

 certain parts of the original primary sympathetic cord; (c) longi- 

 tudinal, the entire series being joined together by two longitudinal 

 bundles of fibers uniting them in a chain. The central connec- 

 tions constitute the rami communicantes , and are as numerous as 

 the sympathetic ganglia themselves; but so close is the approxi- 

 mation of the sympathetic ganglion to the roots of the spinal 

 nerves that they are not visible externally, the ganglion appear- 

 ing to be sessile on the root (Fig. 145); sections, however, show 

 the fibers. The peripheral connections constitute the various 

 nerves of the abdominal viscera; these are not metameric; 

 their number and arrangement is shown in Figure 153. 



In the period between the fourth and the eighth da}^ the pri- 

 mary sympathetic cord becomes resolved into the various ganglia 

 and nerves constituting the aortic plexus, the splanchnic plexus, 

 and the various ganglia and nerves of the wall of the intestine. 

 Remak's ganglion has grown and formed connections with the 

 splanchnic plexus, and other parts of the primary sympathetic 

 cord. The details of these various processes are too complex 

 for full description; they are included in part in Figs. 153 and 154. 



Ganglia and Nerves of the Heart. The development of the 

 cardiac nerves is of special interest on account of its bearing on the 

 physiological problem of the origin of the heart-beat. The heart 

 of the chick begins to beat long before any nervous connections 

 with the central system can have been established; indeed, the 

 rhythmical pulsation begins at about the stage of 10 somites 

 when the neural crest is yet undifferentiated, and no neuroblasts 

 are to be found anywhere. Either, then, the heart-beat is of mus- 

 cular origin (myogenic), or, if of nervous origin, the nerve-cells 

 concerned must exist in the wall of the cardiac tube ah initio. 



The first trace of nerve-cells is found in the heart of the chick 

 about the sixth day. These cells are at the distal ends of branches 

 of the vagus, with which they have grown into the heart. Pre- 

 vious to this time these neuroblasts are found nearer to the vagus 

 along the course of the arteries. There can be but little doubt 

 that they have arisen from the vagus ganglion and that they 

 reach the heart by migration. Such an origin has been demon- 

 strated with great probability for all the known nervous elements 

 of the heart of the chick. (See Wilhelm His, Jr., Die Entwickelung 

 des Herznervensystems bei Wirbelthieren.) 



