68 COMPARATIVE ANATOMY 



ramus communicans. In the anterior part of the body the 

 sympathetic chains lie right and left of and parallel to the 

 vertebral column ; at about the level of the sixth vertebra they 

 approach one another in the middle line, and become closely 

 connected with the dorsal aorta, alongside of which they run. 

 The first sympathetic ganglion lies on the hypoglossal nerve, 

 close to its exit from the first inter-vertebral foramen. Its 

 ramus communicans is represented by a few fine and very 

 short fibres connecting the ganglion with the nerve. From 

 the ganglion two or three nervous strands pass backward, one 

 passing beneath and one or two above the brachial artery, so 

 as to encircle it, as it were, with a ring. This ring is known 

 as the annulus of Vieussens. The second ganglion lies on 

 the brachial nerve ; it is the largest of the whole series, and, 

 like the first, is connected with the spinal nerve by a few fine 

 fibres which do not form a distinct ramus communicans. 

 The third ganglion is usually fused with the second, but has 

 a short and distinct ramus communicans with the third spinal 

 nerve. After this point the sympathetic chain follows the 

 course of the systemic aortic arch, and when this joins its 

 fellow to form the dorsal aorta the sympathetic chains follows 

 the course of the latter. Being thus farther removed from the 

 vertebral column, the rami communicantes become longer. 

 There are as many sympathetic ganglia as there are spinal 

 nerves, and each makes communication by its ramus com- 

 municans with the spinal nerve proper to it. Thus the fourth, 

 fifth, sixth, and successive spinal nerves up to the tenth, 

 supply each a ramus communicans to a sympathetic ganglion, 

 the tenth nerve being peculiar in making several connections 

 as many as ten or twelve in some cases with the sympathetic 

 chain, but the number is not constant. The seventh, eighth, 

 and ninth are also said to have two or three communications 

 apiece with the sympathetic chain. 



The sympathetic chains of each side are connected by 

 numerous fine twigs, which surround the dorsal aorta and 

 form a plexus around it, and from this plexus very fine nerves 

 pass to the adjoining organs. The sympathetic system is 

 characterised by the fact that its branches divide and sub- 

 divide, and the sub-divisions interlace and anastomose with 

 one another to form networks or plexus which include 

 numerous ganglia. The two most important of these are: 



