THE CRANIAL NEEVES. 133 



At the time of opening of the mouth, in tadpoles of about 

 9 mm, length, the condition of the trigeminal nerve is as 

 follows : — The nerve arises on each side, by a single root, from 

 the side of the medulla oblongata ; and, running downwards and 

 forwards, expands to form the Gasserian ganglion, which lies 

 midway between the eye and ear, and immediately in front of 

 the ganglion of the facial nerve. From the ganglion three 

 branches arise : (i) a small but well- marked cutaneous branch 

 runs directly outwards, behind the eye, to the skin, along which 

 it continues forwards for a short distance, (ii) A large ophthalmic 

 hranch runs horizontally forwards between the eye and the 

 brain, parallel to the outer margin of the brain, and dorsal to 

 the optic nerve and to the nose ; in front of the nose it turns 

 slightly downwards, and ends in branches supplying the skin of 

 the snout : the hindmost or proximal part of the ophthalmic 

 nerve is very thick and ganglionic, the distal part is thin, (iii) A 

 very thick mandibular branch, w hich is also ganglionic at its 

 proximal end, runs downwards and forwards below the eye, 

 close to the ganglion of the facial nerve, but separated from 

 this by the anterior cardinal vein ; it runs through the jaw 

 muscles, and ends in the floor of the mouth. From the mandi- 

 bular branch a slende r maxillary branch runs forwards, beneath 

 the eye, and along the upper jaw to the anterior end of the 

 head, where it ends in the skin of the upper lip. 



IV. The mode of development of the fourth cra nial nerve ot 

 the frog has not been determined. 



III. T he third cranial nerve is also very imperfectly known. 

 Its early development has not yet been ascertained. At the 

 time of opening of the mouth, in tadpoles of about 9 mm. 

 length, it is present as a slender nerve, arising from the lower part 

 of the side of the mid-brain, not far from the median plane, and 

 having already the course and relations of the nerve in the adult. 



II. The optic, or second cranial nerv e, will be best dealt with 

 in the description of the development of the eye. The nerve- 

 fibres arise in connection with the retina, and grow inwards 

 along the optic stalk to the brain. 



I. T he olfactory, or first cranial ner ve. The early stages in 

 the development of the olfactory nerve in the frog have not 

 been seen ; there are reasons for suspecting that it is developed in 

 part from the epithelium of the olfactory pit itself, and perhaps 



