MEDULLA OBLONGATA. 439 



exist. The only movements which then follow external irritation 

 are the occasional convulsive motions which are due to reflex action 

 of the spinal cord, and which may be readily distinguished from 

 those of a voluntary character. The animal, under these circum- 

 stances, is to all appearance reduced to the condition of a dead 

 body, except for the movements of respiration and circulation, 

 which still go on for a certain time. The tuber annulare must 

 therefore be regarded as the ganglion by which impressions, con- 

 veyed inward through the nerves, are first converted into conscious 

 sensations; and in which the voluntary impulses originate, which 

 stimulate the muscles to contraction. 



We must carefully distinguish, however, in this respect, a simple 

 sensation from the ideas to which it gives origin in the mind, and 

 the mere act of volition from the train of thought which leads to 

 it. Both these purely mental operations take place, as we have 

 seen, in the cerebrum; for mere sensation and volition may exist 

 independently of any intellectual action, as they may exist after 

 the cerebrum has been destroyed. A sensation may be felt for 

 example, without our having the power of thoroughly appreciating 

 it, or of referring it to its proper source. This condition is often 

 experienced in a state of deep sleep, .when, the body being exposed 

 to cold, or accidentally placed in a constrained position, we feel a 

 sense of suffering without being able to understand its cause. We 

 may even, under such circumstances, execute voluntary movements 

 to escape the cause of annoyance ; but these movements, not being 

 directed by any active intelligence, fail of accomplishing their ob- 

 ject. We therefore remain in a state of discomfort until, on awak- 

 ening, the activity of the reason and judgment is restored, when the 

 offending cause is at once removed. 



We distinguish, then, between the simple power of sensation, 

 and the power of fully appreciating a sensitive impression and of 

 drawing a conclusion from it. We distinguish also between the 

 intellectual process which leads us to decide upon a voluntary 

 movement, and the act of volition itself. The former must precede, 

 the latter must follow. The former takes place, so far as experi- 

 ment can show, in the cerebral hemispheres ; the latter, in the gan- 

 glion of the tuber annulare. 



MEDULLA OBLONG ATA. The last remaining ganglion of the en- 

 cephalon is that of the medulla oblongata. This ganglion, it will 

 be remembered, is imbedded in the substance of the restiform bodv, 



