608 



NEKYOUS SYSTEM. 



muscles of the anus. The fifth anterior sacral and the coccygeal are distributed about 

 the coccyx. 



The posterior branches of the spinal nerves are very simple in their distribution. 

 With one or two exceptions, which have no great physiological importance, these nerves 

 pass backward from the main trunk, divide into two branches, external and internal, and 

 their filaments of distribution go to the muscles and integument behind the spinal column. 



It is farther important to note, as we shall have occasion to do more particularly in 

 connection with the great sympathetic 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 JVerves. 



The nerves which pass out from the cranial cavity present certain differences, in their 

 arrangement and general properties, 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 understand their gen- 

 eral properties and anatomical distri- 

 bution. Many of the cranial nerves, 

 however, are peculiar, either as regards 

 their general properties or in their dis- 

 tribution to parts concerned in special 

 functions. In some of these nerves, the 

 most important facts concerning their 

 distribution have been ascertained only 

 by physiological experimentation, and 

 their anatomy is inseparably connected 

 with their physiology. It would be de- 

 sirable, if it were possible, to classify 

 these nerves with reference strictly to 

 their properties and functions ; but this 

 can be done only to a certain extent, 

 and we must adopt as a basis those 

 divisions recognized in the best works 

 upon anatomy. 



The two classifications of the cranial 

 nerves adopted by most anatomists are 

 the arrangements of Willis and of Som- 

 merring. The first of these is the more 

 common, and in it the nerves are num- 

 bered from before backward, in the or- 

 der in which they pass out of the skull, 

 making nine pairs. 



Anatomical Classification of the 



FIG. 195. Roots of the cranial nerves. 



I. First pair ; olfactory. 

 II Second pair; optic. 



III. Third pair ; motor oculi communis. 



IV. Fourth pair ; patheticus. 



V. Fifth pair; nerve of mastication and trifacial. 

 VI. Sixth pair ; motor oculi externus. 



IX. Glosso-pharyngeal. ) 

 X. PneumoErastric. V Eighth pair. 

 XI. Spinal accessory. ) 

 XII. Ninth pair ; su'blinpual. 



The numbers 1 to 15 refer to branches which will be described 

 hereafter. 



Cranial Jt 



erves. 



First Pair. Olfactory ; the special 

 nerve of smell. 

 Second Pair. Optic ; the special nerve of sight. 



Third Pair. Motor oculi communis ; a motor nerve distributed to all of the muscles 

 of the eyeball, except the external rectus and the superior oblique, to the iris, and to the 

 levator palpebra3. 



