602 Dynamic Theory. 



nerves is large, except in the cetacea. In those having baleen they are 

 few and small, but in the other cetacea there are none at all. Thus the 

 Porpoise, which has a cerebrum highly convoluted, and furnished with 

 a sylvian fissure, an elementary Isle of Reil, and an elementary posterior 

 cornu, is nevertheless destitute of olfactory nerves. Its corpus cal- 

 losum is not large, and the anterior commissure is reduced to a rudi- 

 ment. Its ectorhinal and basirhinal tracts at the base of the cerebrum 

 are still preserved. It is from the ' ' basirhinal tract, " which borders the 

 outside of the mastoid lobe, that the external root of the stem of the ol- 

 factory lobe arises. The inner root, which is the shorter one, arises on 

 the inner face of the hemisphere in front of the optic chiasm. The 

 Hedgehog, among the insectivora, is remarkable for the number of its 

 olfactory nerves. All the gyrencephala, except the quadrumana, pos- 

 sess them in large numbers. In quadrumana the olfactory lobe dimin- 

 ishes and the number of olfactory nerves decreases up to man, in whom 

 the number is not often more than twenty ; less relatively than the 

 others possess. 



As mentioned before, the optic nerves in man are made up of fibres 

 from both the optic lobes, or corpora quadrigemina, and the optic thai- 

 ami. Nevertheless it is shown that the corpora quadrigemina are the 

 sole ganglions of the faculty of vision, and that the optic thalami are 

 simply the organs for the condensation and transmission of the sight 

 and other stimuli to the cerebrum. In animals with large eyes the op- 

 tic nerve and the optic lobes are proportionally large, and vice versa, 

 while no such correspondence is observed in the thalami. In Moles the 

 eye, nerve and optic lobe are the smallest among the mammals. In the 

 Giraffe they are the largest. 



The most remarkable peculiarity of the optic nerves is their decussa- 

 tion, mention of which has been made. In the decussation of the op- 

 tic nerves of mammals, all the fibres do not cross. As shown in fig. 346. 



FIG. 346. Diagram, of the optic chiasm, 

 f<, a. Ends toward the eyes. 

 6, 6. Ends toward the brain. 



the fibres from b divide ; those on the outer side 

 keeping on their own side, and going to the eye of 

 the same side, the middle ones cross over, going to 

 the opposite eye, while still others cross over both 

 before and in the rear, forming a commissure. 

 Those in the front commissure appear to connect the eyes without going 

 to the brain, while those in the rear commissure connect the two halves 

 of the brain without going to the eyes. The commissure fibres in front 

 are not found in all mammals, but those in the rear are generally present 

 in those above the rodents. 



The optic nerve of mammals begins in the embryo as a flat strap, as 



