AMPHIBIA 217 



Between the two layers a space occurs which is lined by the 

 arachnoid membrane formed of flattened cells, and the space 

 is occupied by the arachnoid fluid. 



The cranial nerves are developed as in the Elasmobranch, 

 from the neural crest and the medullary tube. There are 

 ten pairs of cranial nerves. 



The nervus terminalis is represented by a slight nerve in 

 the frog. 1 It was discovered by Judson Herrick and found 

 by Snessarew to end in the anterior commissure, where the 

 fibres of the pair of nerves cross, thus in the original anterior 

 end of the forebrain. 



I. The olfactory nerve arises in the olfactory lobe, and, 

 on reaching the nasal cavity through an opening in the ethmoid 

 cartilage, divides into two branches which spread into a large 

 number of fibres supplying the mucous membrane of the nose. 



II. The optic nerve passes from the optic lobes, forming 

 the optic tract, meets and decussates with its neighbour in the 

 floor of the Illrd ventricle, forming the chiasma, and then 

 each nerve extends freely outwards as the optic nerve. The 

 optic nerve leaves the skull by the optic foramen and, entering 

 the eye, expands into the nervous layer of the retina. 



III. The oculomotor is a motor nerve. It arises in the 

 floor of the mesencephalon, leaves the skull by the oculomotor 

 foramen, which lies behind the optic foramen, and divides into 

 branches supplying the superior, inferior, and internal recti 

 and the inferior oblique muscles of the eyeball. 



IV. The trochlear is a motor nerve. It leaves the brain, 

 after decussating with its neighbour, just behind the optic 

 lobes, pierces the skull by the trochlear foramen, which is 

 situated just above and behind the optic foramen, and supplies 

 the superior oblique muscle of the eye. 



V. The trigeminal passes from the anterior end of the 

 medulla, expands at once into the large Gasserian ganglion, 

 and, leaving the skull by the prootic foramen, divides into 

 two branches, the ophthalmic and the maxillo-mandibular. 

 The ophthalmic passes above the eye structures to supply 

 the skin of the anterior part of the head, and is sensory. The 



1 1909, Herrick, Jour. ofComp. NeuroL and Psych, vol. 19 ; 1910, Snessarew, 

 Anat. Anzeiger, Bd. 37. 



