THE BRAIN AND NERVES. 311 



these there is another delicate membrane called the tunica 

 arachnoides. The dura mater, by its duplications, forms sev- 

 eral processes and sinuses ; the former, by descending be- 

 tween its divisions, serve to secure the brain in its position, 

 and the latter act as reservoirs for the venous blood, thus pre- 

 venting the brain from being injured by any temporary im- 

 pediment in its passage. 



The pia mater closely embraces the brain, and dips into 

 its convolutions. The brain consists of three parts — the 

 cerebrum, the cerebellum, and the medulla oblongata. 



The cerehrum is considerably the largest, and is divided 

 into two hemispheres, each of which closely corresponds 

 with its fellow. 



On cutting into the cerebrum, we find that it consists of 

 two portions — the medullary or white, and the grey or corti- 

 cal part. The latter is mostly situated towards the surface, 

 and the former towards the centre, but both appear to run 

 into each other. Within the hemispheres there appear to be 

 various cavities, canals, and membranes, which, in this work, 

 it is unnecessary to describe. 



The cerebellum, or little brain, is situated behind the 

 cerebrum, than which it is considerably smaller. It appears 

 to consist of medullary and cortical substance mingled to- 

 gether. 



The medulla oblongata, the smallest division, is situated 

 at the base of the brain. It is medullary in its structure, 

 and gives origin to the greater part of the cranial nerves. 

 It is by far the most sensible part of the brain, for whilst 

 portions of the cerebrum have been cut away in some ani- 

 mals without giving any apparent pain, the least pressure on 

 the medulla is productive of injury or death. The brain is 

 largely supplied by means of the carotid arteries with blood, 

 which is returned to the heart by the jugular veins. 



The spinal marrow may be considered as the continuation 

 of the brain, running from the medulla oblongata, throughout 

 the spinal canal, to the tail. It is enveloped by the same 

 membranes as the brain, and continues to the sacrum, where 

 it ends in several nervous cords. Its form is cylindrical, 

 and it has been found to consist of six bands, in the centre 

 of which there is a sort of canal. The nerves arising from 

 the brain and spinal cord, in sheep, are forty pair, ten of 

 which proceed from the brain and the remainder from the 

 cord, and are therefore called the spinal nerves. 



