104 ZOOLOGY. 



grey matter is entirely outside the white matter exactly the opposite 

 condition to that in the spinal cord. 



Owing to the importance of the cerebral hemispheres and cere- 

 bellum as nerve-centres for the regulation of the most complex 

 actions, the main strands of white matter in the brain are those 

 running to and from these organs, as well as between their left and 

 right portions. These may be briefly mentioned here. The right 

 and left cerebral hemispheres are connected to one another by a 

 number of commissures, as such transverse bands of nerve-fibres are 

 called. The largest of these in the rabbit is the great flat corpus 

 callosum, seen on the dorsal aspect when the two hemispheres are 

 gently pressed apart. The postero-ventral continuation of this is 

 called the fornix. Two other smaller ones run in the anterior and 

 posterior walls of the thalamencephalon, and are called the anterior 

 and posterior commissures. Lastly a large middle commissure runs 

 right across the third ventricle, almost filling up the whole cavity 

 (fig. 57). The crura cercbri have- already been mentioned as 

 consisting of great bundles of fibres running to or from the hemi- 

 spheres. Similar bundles running up into the cerebellum from all 

 directions form its anterior, middle and posterior peduncles (all 

 paired): the anterior connecting it with the hemispheres, the posterior 

 with the spinal cord, while the middle ones connect the right and 

 left lateral lobes with one another through the substance of the 

 pons. The fibres of the anterior peduncles decussate in their course 

 through the mid-brain. 



8. Cranial Nerves. The nerves which come off from 

 the brain, or cranial nerves, differ from the spinal nerves, 

 firstly in not showing such distinct metamerism, secondly 

 in not having dorsal and ventral roots, and thirdly in that 

 they are not all mixed nerves, some being purely motor or 

 purely sensory. They are usually counted as forming 

 twelve pairs, and numbered accordingly, it being customary 

 to distinguish the cranial nerves by Roman numerals. 



It is convenient to divide them into four categories. 



A. Purely sensory nerves. 

 i. Olfactory. 

 n. Optic, 

 vin. Auditory. 



B Purely motor (to muscles of eye only), 

 in. Oculomotor, 

 iv. Pathetic or trochlear. 

 vi. Abducent. 



