1890.] Sympathetic Nervous System in Mammals. 21 



mation of cartilaginous vertebral centra, there is no constriction of 

 the main cord (mouse, 17 18 days). Gangliation begins at and after 

 this date, and is due, in the first place, and principally, to the junc- 

 tion of the splanchnic branches ; this causes the accession of a large 

 number of nerve-fibres at the point of entrance, and the consequent 

 persistence of the component cells (which are joined by these nerves), 

 as ganglion cells. Gangliation is caused, secondly, and to a less 

 extent, by the anatomical relations of the sympathetic cord to the 

 bony segments, vessels, &c., which are developed near it, and which, by 

 their growth, cause indentation or constriction of the cord at certain 

 points. 



This view is supported by the evidence obtained from the dissec- 

 tion of human embryos of different ages (3rd, 4th, 5th, and 6th months), 

 where the cord has the form of a band or strip, constricted irregularly 

 at considerable intervals, rather than of a regularly nodulated chain ; 

 and by the evidence derived from the normal adult structure, where 

 the " segmentation " of the sympathetic cord is apparent rather than 

 real. 



The cervical portion of the embryonic sympathetic cord is at first 

 undifferentiated from the main column. Growing with the growth of 

 the neck, it separates at the origin of the vertebral artery, into two 

 unequal parts. The smaller part forms a fibro-cellular cord, which 

 accompanies that artery, and forms the vertebral plexus. The other, 

 or main, portion accompanies the carotid vessels. Growing rapidly, 

 it becomes constricted off from the main sympathetic cord by the 

 formation of a gradually elongating fibro-cellular commissure, and 

 gives rise to the " superior cervical ganglion." This lies alongside the 

 internal carotid artery, and gives off anteriorly a fibro-cellular bundle, 

 which accompanies and is finally lost upon that vessel, as the carotid 

 plexus. When the middle cervical ganglion is present, it may be 

 looked upon as representing a mass of the original cells of the 

 sympathetic cord, which have been included in the growth of the 

 commissure connecting the main cord to the superior ganglion. These 

 parts may be regarded as belonging to the collateral distribution of 

 the sympathetic system, because (1) they are outgrowths from the 

 main cord, and (2) they receive no splanchnic branches directly from 

 the spinal nerves. 



The caudal termination of the sympathetic system is likewise an 

 outgrowth from the main cord. In the youngest embryos in which 

 it is found (rat, 8 days), the cord is lost at the level of the hind 

 limbs ; at a later period of development (rat, 12 days) it reaches 

 further, to the bifurcation of the aorta ; while in still older embryos 

 it can be traced alongside the middle sacral artery for a considerable 

 distance. It is not joined by splanchnic branches, and it is only 

 in an advanced stage of development (rat, 22 days) that trans- 



