52 CLASSIFICATION 



The Brain. The brain is an exceedingly complex organ, 

 and is the centre of sensations and voluntary actions, besides, 

 like the spinal cord, carrying on operations not dependent on 

 the will. It consists of a central axis, which may be regarded 

 as a forward extension of the spinal cord, and of various out- 

 growths from this. The brains of different kinds of backboned 

 animals differ very largely from one another in the extent to 

 which these outgrowths are developed. In the human brain 

 they attain their largest proportions, and the most noteworthy 

 of them (fig. 26) are the cerebellum in the hinder part of the 

 brain, and the two cerebral hemispheres in front of these. It is 

 the hemispheres which are of greatest * interest, for the grey 

 matter forming their outer layer, or cortex, appears to be the 

 ultimate seat of reason, will, and intelligence. It can be mapped 

 out into small areas or centres, which are partly motor, con- 

 trolling special muscles, and partly sensory, concerned with the 

 special sensations. We have, for example, a special motor area 

 concerned with the muscles of the arm, and if this centre be 

 injured, paralysis of the arm follows. Again, a special centre 

 of vision has been recognized, and if this be damaged, the 

 sight is more or less impaired. In fact, what are termed sen- 

 sations of sight, hearing, taste, &c., depend on changes taking 

 place in these centres consequent on nerve impulses which are 

 conveyed to the brain along afferent nerve-fibres. The hemi- 

 spheres are connected together by a broad band of nerve -fibres 

 (corpus callosum), the presence of which is important as regards 

 classification. 



The spinal cord is not only a central organ capable of doing 

 certain regulative work on its own account, but also serves as a 

 channel of communication between the brain and those parts of 

 the body supplied by the spinal nerves. The brain, however, 

 communicates directly with the head and some other parts of the 

 body by means of twelve pairs of cranial nerves which take origin 

 from it. These differ in many ways from spinal nerves. Some of 

 them are composed entirely of afferent fibres, which place sense 

 organs in communication with the brain, e.g. the optic nerve 

 connected with the eye, and the auditory nerve with the internal 

 organs of hearing. Others are entirely composed of efferent 

 fibres, e.g. three pairs of small nerves which do nothing but 

 supply the muscles moving the eyeballs. Others, again, contain 



