878 COMPARATIVE ANATOMY 



the vertebrae immediately anterior to the nerve, with the single excep- 

 tion of the first cervical nerve, which lies between the skull and atlas. 

 We therefore find eight cervical nerves in the neck region, although there 

 are only seven cervical vertebrae. The spinal nerves are then named 

 according to the regions of the spinal column. In man there are twelve 

 thoracic spinal nerves, five lumbar, five sacral, and three to five coccy- 

 geal. 



THE SYMPATHETIC NERVOUS SYSTEM 



There is a great variation in the sympathetic system of various ver- 

 tebrates. The reason for such variation may be accounted for by the 

 differences of the action and function in the different groups of animals, 

 for it is the sympathetic nervous system which is not under the control 

 of the will, but whose work is the regulation or control, either direct 

 or indirect, of the internal organs, glands, blood vessels, respiratory and 

 reproductive organs. This control is brought about by either stimulating 

 or inhibiting the smooth muscle cells in the walls of the blood vessels, 

 so that by the enlargement of the contraction of the blood vessels a 

 greater or a lesser amount of blood may be supplied to any part. It is 

 also of interest to know that the sympathetic nervous system contains 

 sensory fibers, although when these are stimulated, consciousness of 

 such stimulation does not result. 



The sympathetic nervous system is connected with the spinal nerves 

 by the visceral rami. The method of their development has already 

 been discussed in embryology, which should be reviewed at this point 

 (Fig. 337). 



In some of the higher groups of animals, large plexuses are formed 

 in the more important and vital body regions. These have received the 

 special names of cardiac, pelvic, and hypogastric ganglia, while the large 

 one in the abdominal region is known as the solar plexus. 



There is usually a longitudinal sympathetic trunk connecting the 

 chain ganglia of each side, though in the lampreys the chain ganglia are 

 not connected with each other at all, and the sympathetic system is con- 

 fined entirely to the body cavity, while in slightly higher groups there 

 may be extensions from one to the other ganglion. 



Besides the visceral sensory and motor elements which we have 

 been discussing in the sympathetic system, the visceral rami also carry 

 fibers which arise in the ganglion cells of the dorsal ganglion or in the 

 lateral column of the cord itself (Fig. 492). As their axones and 

 dendrites develop, they interlace with both motor and sensory ganglion 

 cells lying in the chain ganglia. Nerve fibers from these then extend 

 out to the viscera, while others run backward in the dorsal and ventral 

 rami of the spinal nerves to reach blood vessels and smooth muscle 

 fibers in the more peripheral regions of the body. These are purely 



