on the Structure, Distribution, and Functions of the Nerves. 43 



these two distinct classes is obtained, in spite of the difficulties 

 which present themselves on minutely displaying the nerves of 

 the face, mouth, throat, and other parts of the human subject, 

 by ascertaining what parts of the organization of an animal are 

 necessary to life and motion ; and what organs are superadded 

 as the animal advances in the scale of existence, as necessary 

 to higher and more complex enjoyments and actions. Where 

 an animal is endowed with mere sensation and locomotion, 

 without a central organ of circulation, and with no organ 

 of respiration but what is generally diffused over the whole 

 frame, the nerves are extremely simple, consisting only of two 

 chords running in the length of the body, with branches going 

 off laterally to the several divisions of the frame. There is no 

 intricacy to be seen, nor can a double supply of nerves be ob- 

 served ; but each portion of the frame has an equal supply : 

 and the central line of connexion is sufficient to combine 

 the actions of the muscles, and to give them the concatena- 

 tion necessary to locomotion. This system is designated by 

 Mr. Bell, the original and symmetrical system, which, he says, 

 exists as well in the human body as in the leech or worm, 

 but is observed by those additional or superadded nerves 

 which belong to those organs which, tracing the orders of 

 animals upwards, are observed gradually to accumulate until 

 we arrive at the complication of the human frame. These 

 additional or superadded nerves, however, do not destroy the 

 original system ; for when we separate certain nerves, it is pre- 

 sented even in the human body. 



This original and symmetrical system is constituted of the 

 nerves of the spine, the tenth or sub-occipital nei've, and the 

 fifth pair of the head or trigeminus of Willis: all which pre- 

 sent a striking agreement with each other with respect to 

 origin, modes of distribution, and function; in that they all 

 have double origins, and ganglia on one of their roots ; they 

 go out laterally to certain divisions of the body; they do not 

 interfere to unite the divisions of the frame ; they are all mus- 

 cular nerves ordering the voluntary motions of the frame ; 

 they are all exquisitely sensible, and the source of the com- 

 mon sensibility of the surfaces of the body; they pervade every 

 part, and yet are symmetrical and simple as the nerves of the 

 lower animals. 



Exposing the nerves of this class in a living animal, they 

 exhibit the highest degree of sensibility, and allow of being 

 distinguished from the nerves of the other class, by the com- 

 parative want of sensibility of the latter. The division of a 

 nerve of this original class is followed by a loss of sensibility 

 to the skin and common substance; whereas these parts arc 



F 2 in 



