706 THE NERVES. 



The external commences by a grey-coloured convolution which borders, 

 externally, the mastoid lobule or inferior lobe of the hemisphere. The 

 internal, followed from before backwards, turns round in the interlobular 

 fissure, in front of the chiasma of the optic nerves, to mix with the cere- 

 bral convolutions. These two roots circumscribe a triangular space oc- 

 cupied by the extra-ventricular nucleus of the corpus striatum, which they 

 embrace. In brains which have been macerated for a long time in alcohol, 

 it is easy to see that the fibres of these roots are continuous, in great part, 

 with those of the corpus striatum, and commence with the fasciculi of the 

 isthmus, which radiate and spread across the grey matter of that body. 



After the union of its two roots, the olfactory lobe is constituted by a 

 wide white band that passes forward on the inferior face of the hemisphere, 

 and soon terminates in a very elongated oval dilatation lodged in the 

 ethmoidal fossa. This bulb is formed by grey substance on its inferior 

 face and white substance on the superior. It is said to be a flattened 

 ganglion applied to a band of white substance, which at first represents in 

 itself the olfactory lobe. 



We have already seen that this lobe is hollow internally, and that it 

 communicates with the lateral ventricles of the brain. This peculiarity, 

 added to the special features of its external physiognomy, might, it appears 

 to us, give rise to doubts as to the real nature of the lobes in question. 

 It is evident that these are not nerves, but rather dependencies of the en- 

 cephalon ; and it is only conformable to custom that we describe them here 

 as the first pair of encephalic nerves. 



The real olfactory nerves are the filaments which arise from the inferior 

 face of the ethmoidal bulb or ganglion, and which traverse the cribriform 

 lamella to gain the mucous membrane of the nose. Their number cor- 

 responds with the ethmoidal foramina. At first very soft, delicate, and 

 easily torn, they are enveloped on their passage through these apertures 

 by a very strong neurilemma, which gives them great solidity. Some, in 

 ramifying, descend on the septum nasi ; others and these are the most 

 numerous divide on the ethmoidal cells, where they form fine and more or 

 less plexuous pencils, mixed up with the no less interesting divisions of the 

 ethmoidal branch of the ophthalmic artery. Their terminal extremities 

 do not descend below the upper third of the nasai fossse, but remain con- 

 fined to the bottom of these cavities. 



These are the special nerves of smell. They receive the impression of 

 odours and transmit them to the encephalon ; and this function, which has 

 been accorded and refused them time after time, has only been decided 

 within the last thirty years. 



2. Second Pair, or Optic Nerves. (Fig. 329.) 



The nerves of vision present for consideration in their interesting study, 

 their oriyin, course, termination, and properties. 



There has been much dispute, and there will probably be much more, 

 with regard to the origin of the second pair. But without confining our- 

 selves to an appreciation of the opinions which have pervaded science on this 

 matter, we will describe what we have observed in the domesticated animals. 



When the encephalic isthmus is isolated from the brain (Fig. 324, 12) 

 and examined laterally, we recognise on its anterior limit the white band 

 that constitutes the optic nerve. Studied at its origin, this band is con- 

 tinued, in the most evident manner, with the external side of the thalamus 



