514 OF THE NERVOUS SYSTEM AND ITS ACTIONS. 



The posterior border of the Optic Chiasma is formed exclusively of com- 

 missural fibres, which pass from one optic ganglion to the other, with- 

 out entering the real optic nerve. Again, the anterior border of the 

 chiasma is composed of fibres, which seem, in like manner, to act as a 

 commissure between the two retinae ; passing from one to the other, 

 without any connexion with the optic ganglia. The tract which lies 

 between the two borders, and occupies the middle of the chiasma, is 

 the true optic nerve ; and in this it would appear that a portion of the 

 fibres decussates, whilst another portion passes directly from each Op- 

 tic ganglion into the corresponding eye. The fibres which proceed from 

 the ganglia to the retinae, and constitute the proper optic nerves, may 

 be distinguished into an internal and external tract. Of these, the 

 external, on each side, passes directly onwards to the eye of that side ; 

 whilst the internal crosses over to the eye of the opposite side. The dis- 

 tribution of these two sets of fibres in the retina of each side respectively, 

 is such that, according to Mr. Mayo, the fibres from either optic gan- 

 glion will be distributed to its own side of both eyes ; the right optic 

 ganglion being thus exclusively connected with the outer part of the 

 retina of the right eye, and with the inner part of the retina of the left 

 eye; and the left optic ganglion being, in like manner, connected ex- 

 clusively with the outer side of the left retina, and with the inner side 

 of the right. Now as either side of the eye receives the images of 

 objects, which are on the other side of its axis, it follows, if this account 

 of their distribution be correct, that in Man, as in the lower animals, 

 each ganglion receives the sensations of objects situated on the opposite 

 sides of the body. The purpose of this decussation may be, to bring 

 the visual impressions, which are so important in directing the move- 

 ments of the body, into proper harmony with the motor apparatus ; so 

 that, the decussation of the motor fibres in the pyramids being accom- 

 panied by a decussation of the optic nerves, the same effect is produced 

 as if neither decussated, which last is the case with Invertebrated 

 animals in general. 



6. Functions of the Cerebellum. 



908. Much discussion has taken place, of late years, respecting the 

 uses of the Cerebellum; and many experiments have been made to 

 determine them. That it is in some way connected with the powers of 

 motion, might be inferred from its connexion with the antero-lateral 

 columns of the Spinal Cord, as well as with the posterior ; and the com- 

 parative size of the organ, in different orders of Vertebrated animals, 

 gives us some indication of what the nature of its function may be. For 

 we find its degree of development corresponding pretty closely with the 

 variety and energy of the muscular movements which are habitually exe- 

 cuted by the species ; the organ being the largest in those animals which 

 require the combined effort of a great variety of muscles to maintain their 

 usual position, or to execute their ordinary movements ; whilst it is the 

 smallest in those which require no muscular exertion for the one pur- 

 pose, and little combination of different actions for the other. Thus, 

 in animals that habitually rest and move upon four legs, there is com- 



