COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 



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to have been the sensory part of the oculomotor nerve, but it has been absorbed by the 

 trigeminus which has spread beyond its original distribution. The deep ophthalmic alone 

 persists in higher forms as the ophthalmic branch of the trigeminus, the superficial ophthalmic 

 branch disappearing. The remainder of the trigeminus is the sensory portion of the troch- 

 lear nerve (see Fig. 69, p. 300) . It is important to note further that the trigeminus is the nerve 

 of the upper and lower jaws. Since the jaws and associated parts constitute the first visceral 

 arch, the trigeminus is said to be the nerve of the first visceral arch. It is the sensory 

 nerve of this arch and also the motor nerve of its visceral muscles. 



g) The seventh or facial nerve: This nerve is intimately related to the tri- 

 geminus. It arises in common with the latter from the anterior end of the me- 

 dulla and divides into three main branches. Two of these branches pass through 

 the orbit in common with the trigeminus. The superficial ophthalmic branch 

 of the facial nerve accompanies and is inextricably mingled with the same branch 

 of the trigeminus. It supplies the supraorbital branch of the lateral line system. 

 The buccal branch of the facial nerve has already been noted as forming part of 

 the infraorbital trunk. Its branches were traced above with the maxillary nerve. 

 It supplies the infraorbital lateral line canal, lying on the ventral side of the 

 rostrum. These two branches of the facial nerve are sensory branches which 

 are lost when the lateral line system disappears. The third branch of the facial 

 nerve is the large hyomandibular trunk which has already been located posterior 

 to the spiracle. Trace it inward toward the brain, cutting tissues in its path. 

 It turns ventrally and runs through the anterior part of the ear capsule deep 

 down. Follow it by removing the cartilage of the ear capsule in small pieces. 

 The nerve passes ventral to some of the branches of the nerve of the ear and 

 joins the anterior end of the medulla in common with the trigeminus root. 

 Near the brain it has an enlargement or ganglion {geniculate ganglion). From 

 this ganglion is given off the palatine nerve. It will be found by dissecting care- 

 fully around and on the ventral surface of the ganglion. In the skate it is easily 

 seen. It runs forward below the orbit along the roof of the mouth where it 

 supplies the taste buds and the lining epithelium in general. Now trace the 

 hyomandibular outward, past the spiracle. It turns ventrally and breaks up 

 into branches on the side of the head. These branches supply the hyoman- 

 dibular and mandibular lateral line canals, ampullae and similar sense organs, 

 the muscles of the hyoid arch, and the lining of floor of the mouth cavity and 

 tongue. 



The hyomandibular is seen to be a mixed nerve with lateral line, visceral motor, and 

 visceral sensory components. The sensory part of the facial nerve is supposed to belong 

 to the same segment of the head as that of which the abducens constitutes the motor nerve. 

 The facial nerve is the nerve of the first gill slit (spiracle) and of the second visceral arch (hyoid 

 arch) and of the muscles belonging to this arch. The lateral line parts of the facial nerve 

 which in fishes compose the greater part of it are lost in land vertebrates, only the palatine 

 and certain parts of the hyomandibular branch persisting as the facial nerve of land verte- 

 brates. 



