THE FROG 



81 



The third nerve, a small motor nerve arising from the ventral surface 

 of the brain between the crura cerebri, is distributed to four of the 

 muscles concerned with moving the eyeball, namely, the superior 

 and inferior recti, the internal rectus and the inferior oblique. It is 

 termed the motor oculi. The fourth, the pathetic, is also a very 

 small motor nerve which, arising from the dorsal surface of the brain 

 between the optic lobes and cerebellum, runs to the superior oblique, 

 another of the eye muscles. The sixth, or abducens, similarly is 



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FIG. 26. Diagram of lateral view of distribution of cranial nerves, 

 Rana, adapted from Howes. 



A., atrium ; B., brachial nerve ; C.A., conus arteriosus ; E., Eustachian tube ; E.N., external 

 naris ; L., lung; N., nasal bone; Oe., oesophagus ; S., squamosal bone; S.V., sinus venosus ; 

 T., transverse process of znd vertebra ; V., ventricle ; V.C., vena cava posterior ; II., optic 

 nerve ; V., main trunk of trigeminal ; V.Mn., mandibular branch of trigeminal ; V.Mx., maxillary 

 branch of trigeminal; V.O., ophthalmic branch of trigeminal; VII., main trunk of facial; 

 VII., hyomandibular branch of facial ; VI I. P., palatine branch of facial ; IX., glossopharyngeal ; 

 IX'., dorsal ramus of glossopharyngeal ; X'., dorsal ramus of vagus ; X.c., cardiac branch of 

 vagus; X.G., gastric branch of vagus; X.L., laryngeal branch of vagus; X. P., pulmonary 

 branch of vagus : I., hypoglossal ; Id., dorsal ramns of hypoglossal ; 2, 3, and 4, second, third 

 and fourth spinal nerves. 



a very small motor nerve, but it takes its origin from the ventral 

 surface of the medulla just behind the pituitary body. It runs to 

 the last of the muscles concerned with the movement of the eye, 

 namely, the external rectus. 



The largest of the cranial nerves is the fifth, or trigeminal. It 

 arises from the side of the medulla by two roots which unite and 

 enlarge to form a swelling, the pro-otic ganglion, where it comes into 

 contact with the sixth and seventh nerves, and then leaving this 

 ganglion it passes through the skull just in front of the auditory 



