ORIGIN OF CRANIAL NERVES. 179 



sulcus, it is prismatic in form, the apex of the prism being directed down- 

 wards (in this position). 



Anteriorly the nervous substance swells into the olfactory bulb, a pyri- 



form grayish mass, about half an inch in length, which rests on the ethmoid 

 bone, and distributes nerves to the nose. 



Posteriorly the olfactory process is connected to the cerebrum by three 

 roots, external, internal, and middle. 



The external or long root is a slender white band, which is directed 

 backwards along the outer part of the anterior perforated space, and across 

 the fissure of Sylvius, and sinks into the substance of the cerebrum. 



The internal or short root, not always visible, is white and delicate, and 

 comes from the inner part of the cerebrum. 



The middle or gray root is connected with the gray matter on the sur- 

 face of the brain by means of a conical elevation at the back of the sulcus 

 which lodges the nerve. 



Deep origin. The external root is said to be traceable to one of the convolutions 

 of the island of Reil. The inner root joins a band of white fibres connected with 

 a convolution (gyrus fornicatus) to be afterwards examined. And the middle root, 

 continuous with the gray matter of the convolutions, contains white fibres which 

 enter the corpus striatum. 



The SECOND or OPTIC ( 2 ) is the largest of the cranial nerves except the 

 fifth, and appears as a flat band on the crus cerebri. Anteriorly the nerves 

 of opposite sides are united in a commissure. The part of the nerve pos- 

 terior to the commissure is named optic tract ; but the part beyond the 

 commissural union, which is round and firm, is called optic nerve. The 

 destination of the nerve is to the eyeball. 



The origin of the nerve will be afterwards seen to come from the optic 

 thalamus and the corpora geniculata, and from one of the corpora quadri- 

 gemina (nates). 



The tract is the flattened part of the optic winding round the peduncle 

 of the cerebrum. In front it ends in the commissure, and behind it splits 

 into two pieces by which it is fixed to the brain. As the tract reaches 

 forwards it crosses the crus cerebri, to which it is attached by its outer or 

 anterior edge ; and in front of the crus it is placed between the substantia 

 perforata antica on the outside, and the tuber cinereum on the inside, but 

 whether it receives filaments from the latter is uncertain. 



The commissure (chiasma) of the nerves measures half an inch across, 

 and lies on the olivary eminence of the sphenoid bone, within the circle 

 of Willis. It is placed in front of the tuber cinereum ; and passing be- 

 neath it (in this position of the brain) is the thin lamina cinerea. 



In the commissure each tract is resolved into three sets of fibres with 

 the following arrangement : the outer fibres, few in number, are continued 

 straight to the eyeball of the same side. The middle, the largest, decus- 

 sate with the corresponding bundle of the other tract, those of the right 

 nerve being continued to the left side, to the opposite eye, and vice versa. 

 And the most internal are continued through the tract of the other side 

 back to the brain without entering the eye. At the front of the commis- 

 sure are placed some transverse fibres, which are prolonged to the eyeballs 

 through the part of each nerve in front of the commissure, but have not 

 any connection with the tracts and the brain. 



The part called nerve extends from the commissure to the eyeball. 

 Leaving the skull by the optic foramen, it receives a tube of dura mater, 



