FROG : NERVOUS SYSTEM AND SENSE ORGANS 73 



the lamina terminalis. Behind this on each side an opening 

 known as the foramen of Monro or foramen interventriculare 

 leads into the cavity or lateral ventricle of one of the cere- 

 bral hemispheres. These are oblong-oval bodies narrowing 

 forwards to join a mass which is indistinctly separated into 

 two olfactory lobes. The median walls of the cerebral 

 hemispheres touch in front and behind, but for a consider- 

 able distance they are quite separate. The brain, like the 

 spinal cord, contains both white and grey matter. In the 

 medulla oblongata and optic lobes the grey matter lies 

 mainly around the ventricles, but in the thalamencephalon, 

 cerebral hemispheres, and olfactory lobes there is an outer 

 grey layer or cortex over the white matter. 



The first or olfactory cranial nerve of each side arises 



from the ' olfactory lobe and runs forward to 

 Serlres'i t * ie olfactory organ in the nostril. The second 



or optic nerve starts from the side of the mid- 

 brain, curves round underneath the brain, running forwards 

 and inwards, and crosses its fellow below the thalamen- 

 cephalon on its way to the eyeball of the opposite side. 

 Where the nerves cross they are fused, and the X-shaped 

 structure thus formed is called the optic chiasmal The 

 third or oculomotor nerve supplies the eye muscles, with the 

 exception of the superior oblique and external rectus. The 

 small fourth, pathetic, or trochlear nerve arises between the 

 optic lobe and cerebellum and supplies the superior oblique 

 muscle. It is the only nerve which starts from the dorsal 

 surface of the brain. The large fifth or trigeminal nerve 

 arises from the side of the anterior part of the medulla. Just 

 before it passes through its foramen it bears a large swell- 

 ing, the Gasserian-geniculate or prootic ganglion. It then 

 divides at once into an ophthalmic branch, which runs forwards 

 along the inner wall of the orbit and supplies the skin of 

 the forepart of the head, and a main branch, which runs 

 outwards across the hinder part of the orbit and divides 

 into a maxillary branch to the upper jaw and a mandibular 



1 For the foramina by which the cranial nerves leave the skull, 

 see p. 32. These nerves can more easily be dissected in the dog-fish, 

 where their course is substantially the same (see pp. 366-370). 



2 The crossing is not complete, part of each nerve proceeding in that 

 limb of the X which passes to the eye of the same side. 



