5i4 STRUCTURE OF VERTEBRATES 



In cyclostomcs the autonomic system is not well differentiated, 

 but in all gnathostomes it is built on much the same plan ; the 

 following description applies primarily to mammals. Physio- 

 logically it is divisible into two, but anatomically into four. The 

 sympathetic system (in the strict sense ; sometimes the term is 

 used as a synonym for the autonomic system) comes from the 

 spinal nerves in the thoracic and lumbar regions (Fig. 472). 

 The axons of the connector neurons emerge with the ventral 

 roots, and then run as white rami communicantes to a segmental 

 series of lateral ganglia on each side of the dorsal aorta ; the gangha 

 are connected by branches of the connector neurons so that they 

 form a double chain. This is the sympathetic system of the frog 

 as it is usually dissected. The chain is continued forwards into 

 the neck where it forms anterior and posterior cervical ganglia, 

 and backwards into the sacral region. In each segment post- 

 ganghonic fibres run as grey rami communicantes to join the 

 appropriate spinal nerves, and supply the sweat glands, erector 

 muscles of the hair, the larynx and the heart, while others run 

 to the viscera, urethra, bladder and copulatory organs. A fibre 

 may run through more than one ganglion, but there is only one 

 synapse between the outflow from the cord and the effector. All 

 sympathetic nerves are adrenergic ; that is, they act on their 

 effectors by liberating a substance similar to, if not identical 

 with, adrenaline. 



The parasympathetic system, by contrast, is choHnergic, that 

 is, it acts by liberating acetylcholine, just as do the somatic 

 motor nerves. It has three outflows from the central nervous 

 system ; through the oculomotor nerve to the ciHary ganglion, 

 and so to the sphincter of the iris and the ciHary muscle ; through 

 the facial, glossopharyngeal and vagus nerves ; and through 

 the spinal nerves of the sacral region. The branches from the 

 facial and glossopharyngeal supply the saHvary glands, and those 

 from the vagus go to the heart, lungs and alimentary canal as 

 far back as the posterior end of the small intestine ; they form 

 the plexus of Auerbach and the plexus of Meissner. The sacral 

 outflow sends branches to the large intestine, bladder and copu- 

 latory organs. Both systems thus supply smooth muscle and 

 glands all over the body, and they are largely antagonistic in 

 their effects ; for example, the sympathetic branch from the 

 inferior cervical ganglion to the heart accelerates it, while the 

 parasympathetic branch of the vagus (the depressor) slows its 



