THE SYMPATHETIC SYSTEM. 



207 



canal, etc. While the sympathetic elements are migrating in 

 this way to their definitive positions, some of the cells of the 

 chain ganglia and perhaps of other ganglia send fibers back along 

 the rami communicantes into the spinal ganglia or the rami 

 of the spinal nerves. The fibers which grow out from the spinal 

 nerves acquire myelin sheaths and so become white fibers, and 

 the part of each ramus communicans formed by them is known as 

 the white ramus communicans. The fibers which grow from the 

 sympathetic cells back into the spinal ganglion or nerve form 

 the gray ramus communicans. 



In selachians a pair of chain ganglia is formed in each segment 

 in the trunk and for some distance into the tail. The anterior six 

 pairs of trunk ganglia disappear during development. This Hoff- 

 mann attributes to the shifting backward of the heart and other 

 organs with reference to the spinal column and nerves. In man 

 the three cervical ganglia which are found in the adult are supposed 

 to be formed by the fusion of a larger number of primary segmental 

 ganglia of the chain. It is important to notice that there is not 

 a complete segmental series of white rami communicantes hi mam- 

 mals. In the cervical segments from which the spinal accessory 

 nerve roots take their origin there are few or no myelinated fibers 

 in the rami communicantes, while in the segments immediately 

 following the last root of the accessory nerve there appears suddenly 

 a great increase in the number of such fibers. A second increase 

 in these fibers begins at the caudal border of the brachial plexus 

 and extends to the beginning of the lumbar plexus. The fibers of 

 the white rami are small myelinated fibers which are excito-motor 

 or excito-glandular in function and are in a broad way serially 

 homologous with the efferent fibers of the vagus and spinal acces- 

 sory nerves. The question of this homology will be taken up 

 again a little later (p. 215). 



The development of the sympathetic system in the head has not 

 been well studied. Some indications as to its source can be 

 obtained by considering the character of the cranial nerves from 

 which it is derived, and from its structure in the most primitive 

 vertebrates. In selachians the development of the ciliary ganglion 

 has been repeatedly described but it is still imperfectly understood. 



