ix.] THE NERVOUS SYSTEM. 391 



These three nerves present a singular constancy (both as 

 to origin and distribution) in Vertebrates generally,"but where 

 there are extra muscles of the orbit, such as have been 

 already described in the Eighth Lesson, these muscles are 

 supplied by the sixth pair of nerves. 



It may be, however, that the whole of these nerves are 

 quite blended with one division (the ophthalmic) of the fifth 

 nerve, as e.g. in the Lepidosiren ; or the sixth nerve alone 

 may form one with the fifth, as in the Bull-frog and others ; 

 or the third and fourth nerves may unite to form a com- 

 mon trunk, -as in the Lamprey, the external and inferior 

 rectus, together with the inferior oblique, being furnished 

 with nerves by the first (ophthalmic) branch of the fifth 

 nerve. Finally, the whole of these three nerves may abort, 

 as in the Myxine. 



22. The FIFTH NERVE, a very large and important one, 

 passes forth from the side of the mid-brain, and, after swell- 

 ing out into what is called the Gasserian ganglion, divides 

 into three nerves, called respectively the ophthalmic, the 

 superior maxillary, and the inferior maxillary. 



The ophthalmic nerve passes through the spheno-orbital 

 fissure. It subdivides into various branches, of which we may 

 note (i), the nasal branch, which skirts the ethmoid, and 

 (2), the lachrymal branch, which passes along the outer side 

 of the orbit to the lachrymal gland. 



The superior maxillary nerve passes through the foramen 

 rotundum and infra-orbital foramen, and goes to the nose, 

 eyelid, upper lip, and teeth of the upper jaw. 



The inferior maxillary nerve passes out through the 

 foramen ovale, and goes to the tongue, salivary glands, and 

 adjacent parts, and, traversing the canal of the mandible, 

 supplies the teeth of the lower jaw with nerve filaments. 



The tongue, although an organ of special sense, will be 

 more conveniently described amongst the other alimentary 

 structures in the Eleventh Lesson. 



A remarkable constancy exists as to the fifth nerve in 

 Vertebrates, except that in some lower forms it blends, as 

 has been said, with the sixth nerve or with all the orbital 

 nerves, and, as we shall find, also more or less with the 

 seventh. The points of exit from the cranium may differ as 

 to details rven in man's own class, as has been indicated in 

 describing the foramina of the skull in the Lesson on that 

 part of the skeleton. 



Comparing the essential characters of this nerve and the 



