THE BRAIN. 417 



CHAPTER IV. 



THE BRAIN. 



BY the brain, or encephalon, as it is sometimes called, we mean all 

 that portion of the nervous system which is situated within the 

 cavity of the cranium. It consists, as we have already shown, of 

 a series of different ganglia, connected with each other by transverse 

 and longitudinal commissures. 



Since we have found the functions of sensation and motion, or 

 sensibility and excitability, so distinctly separated in the spinal 

 cord, we should expect to find the same distinction in the interior 

 of the brain. These two properties have indeed been found to be 

 distinct from each other, so far as they exist at all, in the encephalic 

 mass ; but it is a very remarkable fact that they are both confined 

 to very small portions of the brain, in comparison with its entire 

 bulk. According to the investigations of Longet, neither the 

 olfactory ganglia, the corpora striata, the optic thalami, the tuber- 

 cula quadrigemina, nor the white or gray substance of the cerebrum 

 or the cerebellum, are in the least degree excitable. Mechanical 

 irritation of these parts does not produce the slightest convulsive 

 movement in the muscles below. The application of caustic liquids 

 and the passage of galvanic currents are equally without effect. 

 The only portions of the brain in which irritation is followed by 

 convulsive movements are the anterior surface of the medulla ob- 

 longata, the tuber annulare, and the lower part of the crura cerebri ; 

 that is, the lower and central parts of the brain, containing continu- 

 ations of the anterior columns of the cord. On the other hand, 

 neither the olfactory ganglia, the corpora striata, the tubercula 

 quadrigemina, nor the white or gray substance of the cerebrum or 

 cerebellum, give rise, on being irritated, to any painful sensation. 

 The only sensitive parts are the posterior surface of the medulla 

 oblongata, the restiform bodies, the processus e cerebello ad testes, 

 and the upper part of the crura cerebri ; that is, those portions of 

 the base of the brain which contain prolongations of the posterior 

 columns of the cord. 

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