250 SPECIAL PHYSIOLOGY. 



are very numerous, and pass through minute openings in the ethmoid 

 bone, which forms the roof of the nose; within the upper part of that 

 cavity, they spread out beneath the mucous membrane, and supply 

 branches which advance to its surface to receive the stimulus of odors. 



The second pair, the optic nerves, 2, or nerves of sight, arise on 

 each side, by flat white bands, named the optic tracts, the fibres of 

 which may be traced from the optic thalami, corpora quadrigemina, 

 and corpora geniculata, and, it is said, even from the occipital lobes 

 of the cerebral hemispheres. These tracts pass forward on the sides 

 of, and beneath, the cerebral peduncles, and meet in front, as already 

 mentioned, at the optic commissure; from this, the optic nerves proper 

 are given off in front, and pass through the optic foramina in the 

 sphenoid bone, to enter the bottom of the orbits, whence they proceed 

 forwards to the back of the eyeballs. There, each nerve piercing the 

 thick coats of the eyeball, spreads out in its interior to form the retina, 

 which receives the stimulating impressions of light. At the optic 

 commissure, a portion of the fibres of the optic tract continue into the 

 corresponding optic nerve ; another portion passes over into the optic 

 nerve of the opposite side, thus forming a partial decussation; certain 

 transverse fibres pass from one optic tract to the other, doubtless run- 

 ning back to the brain on each side; whilst other transverse fibres 

 pass from one optic nerve to the other, serving to associate the retinae 

 of the two eyes ; these are not directly continuous with the fibres of 

 the optic tracts. 



The third pair, motores oculi, or motor nerves of the eyeball, 3, arise 

 from the inner surface of the cerebral peduncles, having deep origins 

 from the gray matter there ; a few fibres also spring from the corre- 

 sponding corpora quadrigemina. They enter the orbit, and supply 

 all the muscles of the eyeball, except the superior oblique and the ex- 

 ternal rectus. This nerve also sends branches, called ciliary nerves, 

 which penetrate the coats of the eyeball, and supply the ciliary mus- 

 cle, as well as the circular fibres of the iris. 



The fourth pair, or pathetic nerves, 4, the smallest of the cranial 

 nerves, arise from the back of the cerebral peduncles, and passing 

 forwards, outside the peduncles, a little below the. corpora quadri- 

 gemina, enter the orbit, and supply a single muscle of the eyeball, 

 viz., the superior oblique or trochlear muscle; hence it is also called 

 the trochlear nerve. 



The fifth pair, 5, named the trigeminal nerves, because they divide 

 into three chief branches, and trifacial, because these three branches 

 appear on the face, are the largest of the cranial nerves. They arise 

 from the sides of the pons Varolii, by two distinct roots, viz., a larger, 

 softer root, which enters a crescentic ganglion, called the Cf-asserian 

 ganglion, from which the three great divisions or branches of the nerve 

 are given off; and a smaller and firmer root, which passes beneath the 

 ganglion, and joins the third division only of the nerve. Both the 

 roots of the fifth nerve arise from the gray matter in the pons and 

 medulla oblongata. Of the three divisions or branches, the first or 

 smallest, the ophthalmic, enters the orbit, and there supplies the eye- 

 ball and all its appendages; it gives branches likewise to the mucous 



