NERVOUS SYSTEM 83 



lateral rectus muscle. The abducens is often united with the fifth nerve 

 close to its origin. In some very few forms it is absent. 



An understanding of how these nerves come to supply the muscles 

 they do in the way that they do can be had only from our knowledge 

 of the manner in which these muscles originally develop. The eye mus- 

 cles arise from three myotomes (Fig. 493). One of the somites forms 

 three of the rectus muscles as well as the inferior oblique muscle, while 

 the other two somites form only one muscle each. As the myotome 

 therefore divides into four parts, the oculo-motor nerve is carried into 

 each part. 



The trochlear nerve is the only motor nerve that leaves the dorsal 

 surface of the central nervous system. It has another peculiarity, in that 

 it has a chiasma in the dorsal surface of the midbrain, from which its 

 fibers extend downward to the ganglion cells of the floor of the mid- 

 brain. 



The nerves of the eye muscles are the only somatic motor nerves in 

 the head of the lower vertebrates. 



V, or Trigeminal Nerve. The trigeminal nerve is one of the largest 

 of the cranial nerves, and arises from the antero-lateral angle of the 

 myelencephalon. The trigeminal nerve bears a large semilunar or 

 Gasserian ganglion (Fig. 494), near its origin. This ganglion may be 

 either just inside or just outside the skull. In all higher vertebrates the 

 nerve divides into three main branches, known as the ophthalmic, maxil- 

 lary, and mandibular branches (Fig. 494). In the lower vertebrates the 

 maxillary and mandibular nerves may be united for some distance: In 

 the fishes, the ophthalmic nerve branches, forming a superficial and a 

 profound ophthalmic nerve, both of which are purely sensory. In the 

 higher vertebrates, it is the profound ophthalmic nerve which alone 

 persists. 



The superficial ophthalmic is distributed to the skin, the top of the 

 head, and the tip of the snout. The profound ophthalmic passes between 

 the eye muscles to send branches to the eyelids and conjunctiva, and 

 then extends to the mucous membrane of the nose. It is connected with 

 the ciliary ganglion to which sympathetic fibers also run. Nerves from 

 this ganglion are sent to the iris and to the ciliary muscles of the eye. 

 The ciliary ganglion controls the smooth muscles of the eye. 



The superficial maxillary nerve passes along the margin of the upper 

 jaw, and supplies the face and the teeth. If, however, the profound 

 ophthalmic is 'reduced, the maxillary nerve extends into the region which 

 the profound ophthalmic usually supplies. In the higher vertebrates, 

 the superior maxillary unites with the spheno-palatine ganglion, to which 

 sympathetic fibers also pass. 



The mandibular nerve goes to the lower jaw, passing on the outer 

 side of. Meckel's cartilage (Fig. 494). In all higher forms, where the 

 jaws become ossified, the nerve lies within the bone. This nerve carries 



