NERVOUS SYSTEM. 335 



First Pair, or Olfactory Nerves. 



These arise from the corpora striata, along the posterior borders 

 of which bodies the medullary bands or roots of them may be 

 traced as high up as the middle lobes of the cerebrum. These 

 are the largest of the cerebral nerves : are bulbous at their origin ; 

 pulpy in texture ; and exhibit, when cut into (comparatively to 

 their size) large cavities, which are walled in by a layer of medul- 

 lary matter, inclosed within a thinner one of cortical substance. 

 Tliese cavities some (erroneously) describe as ventricles of the 

 brain: but they evidently belong exclusively to these nerves. 

 They are conical in figure ; are capacious inferiorly, and generally 

 distended with fluid ; but contract upward into small circular 

 canals, which lead through the trunks of the nerves into the an- 

 terior cornua of the lateral ventricles. Girard calls them the 

 ethmoidal sinuses: but, as this would create confusion in our 

 nomenclature, it would be better to name them the olfactory 

 sinuses. This, the tubular portion of the nerve, is sheathed in dura 

 mater. From its abrupt or truncated termination against the 

 cribriform plate of the ethmoid bone, are transmitted numerous, 

 soft, nervous, filaments, which pierce the foramina of this bony 

 plate, and enter the nose ; where they undergo a further sub- 

 division, and are at length widely dispersed over the schneiderian 

 membrane. 



Second Pair, or Optic Nerves. 



If we may be allowed to estimate the importance of nerves by 

 their volume, the optic will hold the third rank among those of 

 the brain, being exceeded in size only by the first and fifth pairs. 

 They take their rise fron the posterior, contracted, and incur- 

 vated parts of the thalami nervorum opticorum. At first they 

 wind forward and downward, around the crura cerebri, which 

 incipient parts of them have obtained the name of tractus optici ; 

 in doing this they approach each other (in which relative course 

 they are singular, all other pairs diverging), and at length form a 

 junction — an intermixture, some say a decussation — of their fibres 

 (in which they are also singular), just below the corpus albican- 

 tium. After this, they become again two separate nerves, pro- 

 ceed downward and forward through the foramina optica, and 

 enter the cavities of the orbits. Here each nerve continues 

 the same oblique course, surrounded by the muscles of the eye, 

 and penetrates the inner, inferior, and posterior part of the eye- 

 ball, within the interior of which it expands and forms the re- 

 tina. In its whole course, it is inclosed within a sheath, pro- 

 longed from the dura mater, forming one more peculiarity in its 

 distribution. 



