446 THE CRANIAL NERVES. 



CHAPTER V. 



THE CRANIAL NERVES. 



IN examining the cranial nerves, we shall find that although they 

 at first seem quite different in their distribution and properties 

 from the spinal nerves, yet they are in reality arranged for the 

 most part on the same plan, and may be studied in a similar 

 manner. 



At the outset, however, we find that there are three of the cra- 

 nial nerves, commonly so called, which must be arranged in a class 

 by themselves ; since they have no character in common with the 

 other nerves originating either from the brain or the spinal cord. 

 These are the three nerves of special sense; viz., the Olfactory, 

 Optic, and Auditory. They are, properly speaking, not so much 

 nerves as commissures, connecting different parts of the encephalic 

 mass with each other. They are neither sensitive nor motor, in 

 the ordinary meaning of these terms ; but are capable of conveying 

 only the special sensation characteristic of the organ with which 

 they are connected. 



OLFACTORY NERVES. We have already described the so-called 

 olfactory nerves as being in reality commissures, connecting the 

 olfactory ganglia with the central parts of the brain. The masses 

 situated upon the cribriform plate of the ethmoid bone are com- 

 posed of gray matter; and even the filaments which they send 

 outward to be distributed in the Schneiderian mucous membrane, 

 are gray and gelatinous in their texture, and quite different from 

 the fibres of ordinary nerves. The olfactory nerves are not very 

 well adapted for direct experiment. It is, however, at least certain 

 with regard to them that they serve to convey the special sensation 

 of smell; that their mechanical irritation does not give rise to 

 either pain or convulsions; and finally that their destruction, 

 together with that of the olfactory ganglia, does not occasion any 

 paralysis nor loss of ordinary sensibility. 



