THE CENTRAL NERVOUS SYSTEM. 99 



SECTION IX. 

 THE AUTONOMIC SYSTEM. 



In contradistinction to skeletal muscle, the unstriped muscle which 

 is found in the walls of the arterioles, digestive tract, uterus, bladder, 

 and elsewhere, is not under the control of the will, though its contrac- 

 tion is regulated by impulses arising in the central nervous system. 

 The nerves which supply these structures, and also those to the secretory 

 glands, form the autonomic system. 



This consists of (1) branches of some of the cranial nerves, including 

 the vagus and the chorda tympani, and of fibres issuing from the 

 anterior roots of the second and third sacral nerves and known as the 

 nervi erigentes; and (2) the sympathetic system, the pre-ganglionic 

 fibres of which leave the spinal 

 cord in the anterior roots of 

 all the spinal nerves from the 

 first thoracic to the fourth 

 lumbar. 



The pre-ganglionic nerve 

 fibres are medullated and 

 small, varying in diameter FIG. 30. -Diagram to show relation between 



pre-ganglionic and post-gangliomc fibres. 



from 2 to 4j*j they are not B> Spinal cord; A> pre .gangiiomc fibre; B, ceil 

 distributed directly to the muS-hb p 8t - ganglionic fibre; D ' UD8tri P ed 

 tissue which they supply, but 



every fibre ends round a nerve cell which lies in a sympathetic 

 or other ganglion ; from this cell a fresh fibre, which is called post- 

 ganglionic and is usually non-medullated, starts and passes to the 

 peripheral tissue (fig. 30). The point at which the pre-ganglionic 

 fibre comes into contact with the dendrites of a ganglion cell 

 forms a cell station or synapse ; and the nervous impulse, issuing 

 from the central nervous system along a pre-ganglionic fibre, normally 

 passes across the synapse to the ganglion cell and then along the 

 post-ganglion ic fibre to the peripheral tissue. 



Although the entire autonomic system is built up on this general 

 plan, namely (1) pre-ganglionic fibre, (2) cell station, (3) post-ganglionic 

 fibre, and (4) nerve ending, the actual anatomical distribution of the 

 fibres and the situation of the cell stations are very varied. The fibres 

 issuing from the brain and from the sacral region of the spinal cord 

 have their cell station close to or actually within the organ which they 

 supply. The fibres of the sympathetic system take a different course. 

 Lying along each sidejof the vertebral column is a chain of ganglia which 

 forms the lateral sympathetic chain (fig. 31). As a rule there is one 



