functions of the Cerebrum. 649 



might announce that this man takes off his hat ; but what is there to show 

 the motive which arises in the intermedial cells ? Is it politeness, or the 

 force of habit and drill ? or because his head is too warm ? 



A study of comparative anatomy shows that the cerebrum is by no 

 moans one of the original vital organs. It is not possessed by the in- 

 vetebrates except in a few of the highest which seem to be transitional 

 between the vertebrates and invetebrates ; and so it is not to be con- 

 sidered even as essential to a certain considerable degree of the nervous 

 co-ordination and condensation which we denominate mentality. It is 

 likewise very limited in extent and function among the lowest of the ver- 

 tebrates, in fact, not appearing at all in the amphijxus. It is a com- 

 paratively latelj' added organ reinforcing and assisting the essential or- 

 gans that were established before it. 



In adult man the cortical or ^ray matter is usually about one-fifth of 

 an inch thick, varying somewhat at different periods of life. It is 

 thickest and the fissures are deepest in middle age. In infancy and old 

 age the fissures are fewer and simpler, and the cortex thinner. 



The fibres in the cerebrum, which radiate from the optic thalami and 

 corpora striata to the cortex, are commissural only, and terminate in 

 the cells of the cerebrum at one end, and those of the thalami and stri- 

 ata at the other. They connect in cells and ganglia with other fibres 

 which continue down the crura cerebri to the periphery of the body. 

 The anatomical relation which the gray matter of the cerebral cortex 

 bears to the basal or central ganglia, precisety corresponds with that borne 

 by the retina, which is only an expansion of the same substance as the 

 cortex, as shown in the development of the eye; and the radiating fibres 

 are in the same relation as the optic nerve. In the development of the 

 embryo, the cortex originates by being detached from the corpora stri- 

 ata on each side, and the retina by being detached from the optic thal- 

 amus. Each is gradually pushed away from the place of origin, the 

 connecting commissural nerve fibres elongating to suit. They are both 

 sensitive to appropriate stimulations, the retina to those of light, the 

 cells of the cortex to those originating in the environment, light, sound, 

 contact, smell, &c. , and also those arising from the agitations of other 

 cortical cells. 



The cerebrum, like the other great departments of the ganglionic cen- 

 tral system, has its sensory connections direct with the external sense 

 organs, from which it receives directly or indirectly all the stimulations 

 by which it is moved and operated, and it also has its own direct motor 

 connections with the various muscles of the bod}', and many of the 

 glands. It has been shown that the spinal cord is by itself a complete 

 machine for receiving a stimulus from the external sense organs, and 

 forwarding it to a muscle, and that the medulla oblongata, the ccivbi-1- 



