420 THE NERVOUS SYSTEM. 



triculum), which are the continuation of the canal that in the 

 foetus extended through the whole length of the spinal cord 

 and brain. They may, therefore, be regarded as the contin- 

 uation of the cerebro-spinal axis or column ; on which, as a de- 

 velopment from the simple type, the cerebellum is placed; and, 

 on the further continuation of which, structures both larger 

 and more numerous are raised, to form the cerebrum (Fig. 142). 



The cerebral convolutions appear to be formed of nearly 

 parallel plates of fibres, the ends of which are turned towards 

 the surface of the brain, and are overlaid and mingled with 

 successive layers of gray nerve-substance. The external gray 

 matter is so arranged in layers, that a vertical section of a 

 convolution, according to Mr. Lockhart Clarke, generally 

 presents the appearance of seven layers of pale and dark ner- 

 vous substance. The structure of the gray matter is that which 

 belongs to vesicular nervous substance (p. 375). 



It is nearly certain that the cerebral hemispheres are the 

 organ by which, 1st, we perceive those clear and more im- 

 pressive sensations which we can retain, and according to which 

 we can judge ; 2dly, by which are performed those acts of 

 will, each of which requires a deliberate, however quick, de- 

 termination ; 3dly, they are the means of retaining impressions 

 of sensible things, and reproducing them in subjective sensa- 

 tions and ideas; 4thly, they are the medium of the higher 

 emotions and feelings, and of the faculties of judgment, under- 

 standing, memory, reflection, induction, and imagination, and 

 others of a like class. 



The evidences that the cerebral hemispheres have the func- 

 tions indicated above, are chiefly these : 1. That any severe 

 injury of them, such as a general concussion, or sudden pres- 

 sure by apoplexy, may instantly deprive a man of all power 

 of manifesting externally any mental faculty. 2. That in the 

 same general proportion as the higher sensuous mental facul- 

 ties are developed in the vertebrate animals, and in man at 

 different ages, the more is the size of the cerebral hemispheres 

 developed in comparison with the rest of the cerebro-spinal 

 system. 3. That no other part of the nervous system bears a 

 corresponding proportion to the development of the mental 

 faculties. 4. That congenital and other morbid defects of the 

 cerebral hemisphere are, in general, accompanied with corre- 

 sponding deficiency in the range or power of the intellectual 

 faculties and the higher instincts. 



Respecting the mode in which the brain discharges its func- 

 tions, there is no evidence whatever. But it appears that, for 

 all but its highest intellectual acts, one of the cerebral hemi- 

 spheres is sufficient. For numerous cases are recorded in 



