536 COMPARATIVE ANATOMY 



anatomically. The vagus nerve, for example, which seems to be a part 

 of the cerebro-spinal system, contains many autonomic fibers connected 

 with the nervous plexuses of the viscera. Moreover, each sympathetic 

 ganglion of the trunk has fiber connexions with a spinal nerve and with 

 the plexuses of the intestine. (Figs. 439, 442, 443) 



Autonomic nerves are connected not only with the digestive and 

 circulatory systems but also with respiratory and urogenital systems, 

 endocrinal and other glands, and the skin, so that there are few parts of 

 the body which autonomic fibers do not reach. Except possibly the 

 autonomic fibers connected with the ciliary muscle of the eye, autonomic 

 nerves, although markedly influenced by the emotions, are not under the 

 control of the will. 



Two kinds of autonomic nerves, sympathetic and parasjrmpathetic, 

 may be distinguished on the basis of their antagonistic action and their 

 different response to drugs. 



A sharp distinction between the sympathetic and parasympathetic 

 fibers cannot be drawn on the basis of function. The sympathetic 

 fibers are usually excitatory; the parasympathetic are usually inhibitory. 

 Most organs of the body have this double innervation and the action of 

 the two kinds of nerves is antagonistic. But some parasympathetic 

 fibers, as for example those in the vagus nerve, are excitatory. 



The two sets of nerves differ in their reaction to atropin and adrenalin. 

 Atropine is stimulating to parasympathetic nerves but not to sympa- 

 thetic. Adrenalin stimulates sympathetic nerves but not parasympa- 

 thetic. Possibly this latter reaction might be expected, since adrenalin is 

 secreted by cells which have their origin from the sympathetic anlagen. 



To the sympathetic group belong the vasomotor and secretory fibers 

 which connect with the sympathetic ganglia of the neck and trunk, from 

 the first thoracic to the fourth lumbar. Parasympathetic fibers are 

 found in the oculomotor nerve connected with the ciliary ganglion, and 

 in the chorda tympani and vagus nerves, and in the second, third, and 

 fourth sacral nerves. In this system must also be grouped all the neurons 

 with which these nerves have synaptic connexions. 



It is evident, therefore, that there are three distinct groups of auto- 

 nomic nerve fibers, a cranial group, a thoracico-lumbar group, and a sacral 

 or pelvic group. Of these, the cranial and sacral elements are para- 

 sympathetic, and hence are grouped together as the craniosacral division 

 of the autonomic system. The thoracico-lumbar division constitutes the 

 sympathetic portion of the autonomic system. 



Autonomic nerves may also be classified, on the basis of their distribu- 

 tion, into somatic fibers which innervate the blood vessels of the body 

 wall and the smooth muscles of skin and sweat glands, and visceral 

 fibers which supply the glands and smooth muscles of the viscera. 



