418 THE BRAIN. 



The most central portions of the nervous system, therefore, and 

 particularly the gray matter, are destitute of both excitability and 

 sensibility. It is only those portions which serve to conduct sen- 

 sations and nervous impulses that can be excited by mechanical 

 irritation ; not the ganglionic centres themselves, which receive and 

 originate the nervous impressions. 



"We shall now ^tudy in succession the different ganglia of which 

 the brain is composed. 



OLFACTORY GANGLIA. These ganglia, which in some of the 

 lower animals are very large, corresponding in size with the ex- 

 tent of the olfactory membrane and the acuteness of the sense of 

 smell, are very small in the human subject. They are situated on 

 the cribriform plate of the ethmoid bone, on each side of the crista 

 galli, just beneath the anterior lobes of the cerebrum. They send 

 their nerves through the numerous perforations which exist in the 

 ethmoid bone at this part, and are connected with the base of the 

 brain by two longitudinal commissures. The olfactory ganglia 

 with their commissures are sometimes spoken of as the " olfactory 

 nerves." They are not nerves, however, but ganglia, since they are 

 mostly composed of gray matter ; and the term " olfactory nerves" 

 can be properly applied only to the filaments which originate from 

 them, and which are afterward spread out in the substance of the 

 olfactory membrane. 



It has been found difficult to determine the function of these 

 ganglia by direct experiment on the lower animals. They may be 

 destroyed by means of a strong needle introduced through the bones 

 of the cranium ; but the signs of the presence or absence of the 

 sense of smell, after such an operation, are too indefinite to allow us 

 to draw from them a decided conclusion. The anatomical distribu- 

 tion of their nerves, however, and the evident correspondence which 

 exists, in different species of animals, between their degree of de- 

 velopment and that of the external olfactory organs, leaves no doubt 

 as to their true function. They are the ganglia of the special sense 

 of smell, and are not connected, in any appreciable degree, with 

 ordinary sensibility, nor with the pcoduction of voluntary move- 

 ments. 



OPTIC THALAMI. These bodies are jjot, as their name would 

 imply, the ganglia of vision. Longet has found that the power of 

 sight and the sensibility of the pupil both remain, in birds, after 



