THE NERVOUS SYSTEM 525 



afferent nerves are either absent or under normal conditions 

 blunted in sensibility. Every autonomic fibre must somewhere 

 in its course, and before distribution to its tissue, pass into a 

 ganglion, arborise around a nerve-cell, and from this cell derive 

 its axon, which passes to the tissue concerned. There is no 

 exception to this rule ; it is a fundamental principle in the 

 construction of this peculiar system. Every fibre has therefore 

 a cell-station ; it may be in the vertebral, prevertebral, or terminal 

 system. It cannot be in more than one of these, though in the 

 case of those fibres ending in the terminal ganglia they have to 

 pass through the ganglia of the other systems before they reach 

 their destination, but they form no cell connection with them. 

 We are indebted to the labours of Langley for determining the 

 cell-station of the autonomic fibres, which he found could be 

 ascertained by the aid of nicotine. This alkaloid has the effect 

 of paralysing nerve-cells, but not nerve-fibres. In order to 

 ascertain whether a nerve-fibre has its cell-station in any given 

 ganglion, it is only necessary to paint the ganglion with nicotine. 

 If the nerve then on stimulation continues to function, it is certain 

 that, though passing through the ganglion, it has no cell-connec- 

 tion with it. A fibre before it makes a cell-connection is known as 

 a pre-ganglionic ; after it has made its cell-connection it is termed 

 post-ganglionic. Every pre-ganglionic fibre must have originated 

 in the mid-brain, bulb, or cord ; so that every autonomic fibre 

 consists of two neurones, the first lying between the cell of 

 origin in the above tissues and the autonomic ganglion in which 

 its cell-station exists, and the other between the latter and the 

 tissue supplied. In other words, the pre-ganglionic and post- 

 ganglionic fibres represent the two neurones in the autonomic 

 chain, the former being medullated, the latter as a rule non- 

 medulla ted. The primary division of the autonomic system is 

 into Cranial and Spinal. 



Cranial Autonomic System. — It has been stated that there 

 are functions carried out by some branches of the cranial nerves 

 identical in character with those of the sympathetic ; this has 

 led to them being included in the new autonomic system. These 

 nerves are fibres derived from the third, seventh, ninth, tenth, 

 and eleventh pairs. The pre-ganglionic fibres of the third pair 

 arise from the mid-brain, the others from the bulb. The cell- 

 station of the third pair lies in the ciliary ganglion ; here the 

 fibres form synapses, and emerge as post-ganglionic fibres in the 

 short ciliary nerves supplying the plain muscle (sphincter) of the 

 iris, and of the ciliary muscle. The pre-ganglionic fibres of the 

 seventh and ninth pairs have their cell-station in the spheno- 

 palatine, otic, submaxillary, and sublingual ganglia. The post- 

 ganglionic fibres supply dilator fibres to the bloodvessels of 



