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 day 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 cardiae tube ab 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.) 
