472 PHYSIOLOGY 



are situated in the sixth and seventh lumbar and first sacral ganglia. They 

 convey : 



Vase-constrictor impulses. 



Secretory nerves to the sweat glands. 



Every fibre of the sympathetic system is thus in some point <>i its course 

 interrupted by a nerve cell, and Langley has shown that this is the only cell- 

 break in the fibre. This applies not only to the sympathetic fibres but also 

 to the fibres of the other visceral nerves. Each fibre therefore can be re- 

 garded as made up of two sections a pre-ganglionic fibre arising in the 

 central nervous system and passing down to a ganglion as a fine medul- 

 lated nerve fibre, and a post-ganglionic fibre arising in this ganglion 

 and continued generally as a non-medullated fibre to its peripheral dis- 

 tribution. 



This transference from one system to another involves the passage across 

 a synapse and a nerve cell. The situation of this nerve cell may be easily 

 revealed by utilising the action of nicotine, first studied by Langley. If 

 nicotine be applied to a sympathetic ganglion, it first stimulates and then 

 paralyses any junction between axon termination and nerve cell which may 

 lie in the ganglion. Intravenous injection of nicotine therefore causes a 

 primary general excitation of all visceral ganglion cells. There is an enor- 

 mous rise of blood pressure, which may be accompanied by other sympathetic 

 effects, such as dilatation of the pupil, secretion of saliva, erection of the 

 hairs, and so on. This rise rapidly passes off, and it is then found impossible 

 to evoke any reflex visceral effects or any contraction, e.g. of the blood vessel s. 

 by stimulation of the spinal cord ; the passage of the impulses is blocked in 

 every one of the visceral ganglia. *By observing the effects of stimulation of 

 a nerve before it enters a ganglion and then painting the ganglion with nico- 

 tine and again trying the effects of excitation, it is easy to determine whether 

 the nerve fibres which were excited in the first case form any connections 

 with the nerve cells of the ganglion. In the strength in which it 'is usually 

 applied nicotine is without effect on nerve fibres. 



THE CRANIAL AUTONOMIC FIBRES 



In the course of development the greater part of the fibres of the facial 

 nerve have lost their special visceral functions and have the aspect and 

 functions of ordinary somatic fibres. Visceral fibres are contained in the 

 following cranial nerves the third, seventh, ninth, tenth, and eleventh. 



Third nerve. The visceral fibres of this nerve pass to the ciliary ganglion 

 in the orbit where they end ; the post-ganglionic fibres from this ^million 

 form the short ciliary nerves which innervate the sphincter pupill* and the 

 ciliary muscles. 



Seventh nerve. The autonomic fibres of the facial arise from the medulla 

 in the intermediate nerve of Wrisberg, which is practically the anterior con- 

 tinuation of the ninth, tenth, and eleventh nerves. From the seventh nerve 

 is derived the chorda tympani nerve, which supplies vaso-dilator nerves 



