BULB OF THE OLFACTORY NERVE. 593 



This is called the accessory root of Rolando. It is particularly 

 well developed in the bullock. This nerve sends a large root 

 to the ophthalmic ganglion,, from which the ciliary nerves are 

 derived. It appears to be the motor nerve of the iris, for when, 

 in the experiments of Mr. ]\fayo, the third pair was irritated, the 

 iris was observed to contract. 



— The optic or second nerve, the largest of all those of the 

 cranium, tracing it from its chiasm backwards, (where the 

 inner half of the fibres of each nerve seem to cross over to the 

 opposite side,) winds round the crus cerebri, to which it is 

 attached by a thin medullary lamen at its outer edge. As it 

 reaches the thalamus the nerve is divided into two roots, one 

 of which arises from the coi'jms geniculatum externum of 

 that organ ; the other passes over the corpus geniculatum 

 internum to which it is slightly connected, to arise by two slips 

 from the tubercula quadrigemina — one from the nates, and the 

 other from the testes. This latter is the principal origin of the 

 nerve in the inferior animals ; hence the tuberculi quadrigemina 

 have been called the optic tubercles. 



— The olfactory or first pair of nerves, appears to originate 

 by three roots, near the fissure of Sylvius, close to the substance 

 of the corpus striatum. One of these is connected with the 

 anterior commissure of the middle ventricle ; and one with the 

 lateral prolongation of the corpus callosum at this part ; the 

 third or central root comes from a papillary eminence of the 

 cineritious structure in the posterior part of the fissure of Syl- 

 vius. The nerve formed by the union of these roots runs out 

 in a triangular groove on the lower surface of the anterior 

 lobes, to its bulb, by the side of the crista galli of the ethmoid 

 bone. The structure of the bulb is cineritious and pulpy 

 in man. In the inferior animals, as the horse and sheep, and 

 in the human foetus, it is of a similar structure, and forms a 

 large olfactory lobe with a ventricle within it, on the anterior 

 and inferior face of the brain. It would therefore in man be 

 more correct, as Blainville and Rolando have observed, to 

 consider the bulb as an olfactory lobe or ganglion, from which 

 the true olfactory nerves are spread into the Schneiderian 

 membrane, and the grayish cord extending back from it and 

 50* 



