^0 Analogy bet'wee7i Vegetables and Animals. 



sensible to the stimulus of food, as the eye is to that of light. 

 Are we, then, from these circumstances, to conclude that there 

 are two different classes of nerves, one of which exercises the 

 functions of perception and volition, while the other possesses 

 only the power of transmitting nervous influence, and is in- 

 capable of exciting perception. Mr. C. Bell, Majendie, and 

 other physiologists have proved by innumerable facts and 

 direct experiments, that there are two classes of nerves, per- 

 fectly distinct from each other. The first class is called the 

 symmetrical, and consists of the fifth cerebral, and all the 

 spinal nerves: they arise by double roots, and are ramified 

 upon the voluntary muscles : they constitute the organs of 

 perception and volition, and form the means whereby the 

 animal body is connected with the external world. The se- 

 cond class of nerves is termed the irregular, and it includes 

 what are called the respiratory and the ganglionic nerves^ 

 They arise, by single roots, from the base of the medulla 

 oblongata [the medullary substance lying within the cranium 

 or skull, which is the beginning of the spinal marrow], and 

 are distributed to the conservative organs of the body, such as 

 the stomach, the lungs, the heart, &c., which they connect in- 

 timately together. They unite with the symmetrical nerves, 

 and form the bond of union between the two halves of the 

 body. Their property is to transmit the nervous influence 

 from one part to another ; but they do not possess the faculty 

 of communicating either perception or volition. From this 

 account it appears that there are no less than five distinct 

 sets of nerves. For instance, the symmetrical or regular 

 class consists of nerves of sensation and of nerves of motion. 

 If a nerve of sensation be pricked or irritated, great pain is 

 produced ; and, if the same nerve be divided, all sensation is 

 destroyed, but the function of motion is not in the least 

 impaired or affected : on the contrary, if a nerve of motion 

 be lacerated or injured, the muscles upon which its fila- 

 ments are ramified become convulsed ; but, if a motific [mo- 

 tion-causing] nerve be fairly cut in two, the motion of the 

 parts to which it is distributed is entirely lost, but their sens- 

 ation, or power of feeling, remains perfect and natural. The 

 irregular class consists of the nerves distributed to the muscles 

 concerned in the function of respiration ; and of the gan- 

 glionic nerves, which, as I have observed above, are con- 

 ducive to growth and nutrition. Besides these four sets, there 

 are the nerves of the external senses, such as the nerves of 

 touch, the olfactory, the optic, the auditory, and the gustatory 

 nerves. 



{To be continued.') 



