THE CRANIAL NERVES. 449 



bundle only at a certain distance from their point of origin. The 

 mere fact that two nerves leave the cavity of the cranium by the 

 same foramen does not indicate that they have the same or even a 

 similar function. Thus the facial and auditory both escape from 

 the cavity of the cranium by the foramen auditorium intern um, and 

 yet we do not hesitate to regard them as entirely distinct in their 

 nature and functions. It is the ultimate distribution of a nerve, 

 and not its course through the bones of the skull, that indicates 

 its physiological character and position. For while the ultimate 

 distribution of any particular nerve is always the same, its arrange- 

 ment as to trunk and branches may vary, in different species 

 of animals, with the anatomical arrangement of the bones of the 

 skull. This is well illustrated by a fact first pointed out by Prof. 

 Jeffries Wyman 1 in the anatomy of the nervous system of the 

 bullfrog. In this animal, both the facial nerve and motor oculi 

 externus, instead of arising as distinct nerves, are actually given 

 off as branches of the 5th pair ; while their ultimate distribution is 

 the same as in other animals. All the motor and sensitive nerves 

 distributed to the face are accordingly to be regarded as so many 

 different branches of the same trunk ; varying sometimes in their 

 course, but always the same in their ultimate distribution. 



The motor nerves of the head are in all respects identical in their 

 properties with the anterior roots of the spinal nerves. For, in the 

 first place, they are distributed to muscles, and not to the integu- 

 ment or to mucous membranes; secondly, their division causes 

 muscular paralysis; and thirdly, mechanical irritation applied at 

 their origin produces muscular contraction in the parts to which 

 they are distributed, but does not give rise to a painful sensa- 

 tion. In several instances, nevertheless, the motor nerves, though 

 insensible at their origin, show a certain degree of sensibility when 

 irritated after their exit from the skull, owing to fibres of com- 

 munication which they receive from the purely sensitive nerves. 

 In this respect they resemble the spinal nerves, the motor and 

 sensitive filaments of which are at first distinct in the anterior 

 and posterior roots, but afterward mingle with each other, on 

 leaving the cavity of the spinal canal. 



The three sensitive nerves originating from the brain are the 



1 Nervous System of Rana pipiens ; published by the Smithsonian Institution. 

 Washington, 1853. 

 29 



