NERVOUS TISSUE 137 



as may readily be shown in dissections of the adult. In embryos of 10-12 

 mm. these rami are present as short branches, which can be followed to the 

 muscular condensations derived from the myotomes, but apparently at 

 that stage they do not enter the skin. The ventral rami are longer. Most 

 of them anastomose with the ventral rami of adjacent nerves, thus giving 

 rise to the cervical, brachial and lumbo-sacral plexuses. They are dis- 

 tributed to the muscles and skin of the ventral body wall. 



DEVELOPMENT OF THE SPINAL SYMPATHETIC SYSTEM. 



In mammalian embryos measuring 10-12 mm., each of the thoracic 

 spinal nerves exhibits a branch directed toward the aorta, and ending in a 

 rounded mass of ganglion cells. This is the sympathetic or visceral ramus, 

 terminating in a sympathetic ganglion (Fig. 125, E). It is generally 

 believed that the cells in the sympathetic ganglia migrate outward from 

 those in the spinal ganglia, but an origin from cells of the medullary tube 

 which wander out along the ventral roots has also been asserted. Al- 

 though the cells of the sympathetic ganglia were formerly considered to be 

 mesodermal (even after it had been shown that those of the spinal ganglia 

 were ectodermal), it is now generally admitted that the entire sympathetic 

 system is ectodermal. However, in the cervical region the spinal nerves 

 at first do not have sympathetic rami, and the sympathetic ganglia con- 

 sequently appear isolated in the mesenchyma. Their cells may have mi- 

 grated in detached groups. Instead of eight ganglia on either side of the 

 neck, corresponding in number with the spinal nerves, there are but three, 

 known as the superior, middle and inferior cervical ganglia, respectively, 

 and of these the middle ganglion may be merged with the superior. They 

 are elongated structures, especially the superior ganglion, and presumably 

 represent a fusion of segmental ganglia. 



Each sympathetic ganglion in the thorax of the adult is connected 

 with its spinal nerve by two rami communicantes, known as the white and 

 gray rami, respectively. The white rami consist chiefly of fibers passing 

 outward from the spinal nerve, and they are probably a persistence of the 

 sympathetic rami of the embryo. The gray rami contain fibers passing 

 from the sympathetic ganglia back to the spinal nerves, and apparently 

 arise later. They are found not only in the thorax and abdomen, but 

 also in the neck where, as usually described, they place the superior cervi- 

 cal ganglion in connection with the first four cervical nerves, the middle 

 cervical ganglion in connection with the fifth and sixth, and the inferior 

 in connection with the sixth, seventh and eighth. The succession of 

 sympathetic ganglia on either side of the body, extending from the neck 

 to the pelvis, become connected with one another through bundles of 



