124 NERVOUS SYSTEM. 



pathetic nerve, that all of the cerebro-spinal nerves anas- 

 tomose with the sympathetic. This anatomical connection 

 between the two systems of nerves has great physiological 

 interest. 



Cranial Nerves. 



The nerves which pass out from the cranial cavity present 

 certain differences in their arrangement and general proper- 

 ties from the ordinary spinal nerves. As we have seen, the 

 spinal nerves are exceedingly simple, each one being formed 

 by the union of a motor and a sensory root. The function of 

 most of them follows as a matter of course when we under- 

 stand their general properties and anatomical distribution. 

 Many of the cranial nerves, however, are peculiar, either as 

 regards their general properties or in their distribution to 

 parts concerned in special functions. In some of these 

 nerves, the most important facts concerning their distribu- 

 tion have only been ascertained by physiological experimen- 

 tation, and their anatomy is inseparably connected with 

 their physiology. It would be desirable, if it were possible, 

 to classify these nerves with reference strictly to their prop- 

 erties and functions ; but this can be done only to a certain 

 extent, and we must adopt as a basis those divisions recog- 

 nized in the best works on anatomy. 



The two classifications of the cranial nerves adopted by 

 most anatomists are the arrangement of Willis 1 and of Som- 

 mering. 3 The first of these is the more common, and in it 

 the nerves are numbered from before backward in the order 

 in which they pass out of the skull, making nine pairs. 3 



1 WILLIS, Cerebri Anatome : cut accessit Nervorum Descriptio et Usm, Lon- 

 dini, 1664, p. 145, et seq. 



2 SOMMERING, De Basi Encephali et Originibus Nervorum, Goettingae, 1778, 

 p. 69, et seq. 



3 Haller adopted the classification of Willis, and his example has been fol- 

 lowed by nearly all of the later anatomical and physiological writers, but he dis- 

 cards the tenth pair, the suboccipital, or first cervical nerve, originally reckoned 

 by Willis with the cranial nerves (HALLER, Elementa Physiologice, Lausannae, 

 1762, tomus iv., p. 240.) 



