THE KERVOUS SYSTEM. 630 



divided in a pigeon, inco-ordination occurs, with a constant movement 

 of the head from side to side, and similarly, when one of the vertical 

 canals is operated upon, up and down movements of the head are ob- 

 served. The bird is unable to fly in an orderly manner, flutters and 

 falls when thrown into the air, and, moreover, is able to feed with 

 difficulty. Hearing remains unimpaired. So that inco-ordination 

 depends upon deficiency or disorder of normal ampullar influences. It 

 will be recollected that the semicircular canals are supplied with a 

 nerve, the vestibular branch of the auditory, which is connected with the 

 bulb. 



It is probable that the various afferent impulses upon which co-ordina- 

 tion and the maintenance of the equilibrium depend are gathered up, as 

 it were, in the tegmental system from the bulb upward, since this 

 region is so intimately connected with the bulb and cord posteriorly, 

 and with the optic thalamus and corpora quadrigemina anteriorly. In 

 addition to the tegmentum, however, the cerebellum and pons are in 

 some way concerned, because of their intimate connection with the 

 spinal cord and bulb, the cerebellum being further connected with the 

 auditory nerve on the one hand, and with the gray matter in-connection 

 with the tegmentum on the other hand. 



Sensory Centres. 



There is evidence that fibres from the nerves of special sense are 

 specially connected with definite and distinct parts of the cerebrum. 



Visual or Optic Centre. The termination of the optic nerve in each 

 eye, the retina, to the structure of which we shall return when treating 

 of the eye, is so arranged that when we look at an object with both 

 eyes symmetrical parts of each retina are used. For example, if we look 

 at an object to the left, an image of that object is focussed upon the 

 right half of both retinae, viz., upon the temporal side of the right 

 retina, and upon the nasal side of the left retina. The optic nerve- 

 fibres of these symmetrical parts of the retina are gathered together 

 behind where the optic nerves decussate, viz., in the optic chiasma. 

 The fibres which come from the right side of both eyes are contained in 

 the optic tract of the same side, viz. , the right, those from the right eye 

 being outside of the others. In the same way the left optic tract con- 

 tains internally fibres from the left side of the right eye and externally 

 those from the left side of the left eye. On the inner border of the optic 

 chiasma and tract there are also commissural fibres which pass from one 

 side of the brain to the other; these are fibres which connect one median f 

 corpus geniculatum with the other. They are called the inferior or 

 arcuate commissure. The optic tract thus formed then passes back- 



