74 ANATOMY OF THE RABBIT 



Thus the grey ramus communicans differs from the other branches 

 in carrying fibres to rather than from the trunk of the nerve and 

 in that sense might better be designated as a root. 



The Autonomic Nervous System 



UnHke the somatic nerves, which take a direct course to their 

 terminations, the communicating rami of each side unite in a 

 position ventral to the vertebral column to form a longitudinal 

 sympathetic trunk consisting of a connected series of ganglia. Of 

 these trunk ganglia there are on each side two in the neck of the 

 rabbit and a segmented series in the thoracic, lumbar, and sacral 

 regions. The sympathetic trunk is similarly connected with an 

 unpaired collateral series of ganglia, and through them with certain 

 peripheral ganglia on the surface of the visceral organs. From 

 these ganglia, fibres run to the visceral muscles and glands through 

 plexuses which mostly accompany the blood-vessels. The longi- 

 tudinal trunks and their connections with the thoracic and lumbar 

 nerves form the sympathetic portion of the autonomic division of 

 the peripheral nervous system. The corresponding connections 

 of the sacral and certain cranial nerves (the third, seventh, ninth, 

 tenth, and eleventh) constitute the parasympathetic portion of 

 this division. The latter does not join the sympathetic trunk, 

 and its fibres end only in the peripheral ganglia. The two divisions 

 differ also in their responses to certain drugs. Most organs con- 

 trolled by the autonomic system receive fibres from both, which 

 usually produce opposite eiTects upon stimulation though in some 

 cases they co-operate, acting upon different components of the 

 organ. (For instance, mucous and serous cells in the submaxillary 

 gland have respectively parasympathetic and sympathetic control.) 

 It is usual to consider that only the visceral efferent elements 

 constitute the autonomic division, though visceral afferent fibres 

 run through the trunks and ganglia along with the efferent com- 

 ponents and are thus included in the gross anatomical structures. 

 The cell bodies of the afferent neurons, however, lie in the dorsal 

 root ganglia of the spinal nerves as indicated in Fig. 39. 



From the foregoing, it is apparent that the visceral efferent 

 peripheral pathway, unlike the somatic one, always involves two 

 successive neurons. One fibre springs from a cell body in the 



