146 PHYSIOLOGY. 



left nerve to proceed to the luft eye ; so that each eye receives 

 its optic nerve from both sides of the brain ; the right sup. 

 plying the right side of both eyes, as the left does its cor 

 responding side of both. After the nerve has entered tha 

 orbit, it proceeds forwards to the ball of the eye, and pene-. 

 trating the coats of that organ, expands into a very delicate 

 web, or nervous net-work, called the retina, which Jserves to 

 receive the image of objects falling upon it.) 



"5. The auditory nerve. jf he last nerve of specific sense, 

 runs along with another, the two together being known as 

 the seventh, of which the auditory forms the soft portion, or 

 portio mottis. It arises from the medulla oblongata, and 

 goes on in contact with the hard portion, or portio dura, to 

 the internal auditory passage, which they enter. Here the 

 soft portion spreads out into a great many small branches, 

 which are distributed over the surface of the internal por- 

 tions of the ear, and their extremities float in the water 

 contained in the labyrinth, from which they receive vibra- 

 tions which produce the impressions of sound) 



6- Second order. -The motif erous nerves^ which consti- 

 tute the second order, consist of three nerves ; the third, the 

 sixth, and the ninth, with perhaps a branch of the fifth. 

 These take their origin at the anterior columns, in the same 

 line with each other ; but fibrils of sensation afterwards join 

 them, so that they are analogous to the double-rooted nerves 

 of the spinal marrow. 



76^Fhe third pair rise from the crura of the brain, and 



pass forward to a hole in the bottom of the orbit, through 



which they enter, and supply four of the six muscles which 



/move the eye, and also the muscle which moves the upper 



' eye-lid, thus placing these muscles under the command of 



the wilj. Besides this, a small twig of the fifth nerve swells 



out into a ganglion, and then joins a twig from the third, 



which go on together, and penetrate the coats of the eye, to 



which they impart sensibility, and are connected with the 



functions of the irisJ It is worthy of remark, that here the 



