408 THE SPINAL CORD. 



case, confusion would constantly arise, both in the perception of 

 sensations, and in the execution of movements. 



4. There are certain sensations which are excited simultaneously 

 by the same causes, and which are termed associated sensations ; and 

 there are also certain movements which take place simultaneously, 

 and are called associated movements. In the former instance, one of 

 the associated sensations is called up immediately upon the percep- 

 tion of the other, without requiring any direct impulse of its own. 

 Thus, tickling the soles of the feet produces a peculiar sensation 

 at the epigastrium. Nausea is occasioned by certain disagreeable 

 odors, or by rapid rotation of the body, so that the landscape seems 

 to turn round. A striking example of associated movements, on 

 the other hand, may be found in the action of the muscles of the 

 eyeball. The eyeballs always accompany each other in their lateral 

 motions, turning to the right or the left side simultaneously. It is 

 evident, however, that in producing this correspondence of motion, 

 the left internal rectus muscle must contract and relax together 

 with the right external; while a similar harmony of action must 

 exist between the right internal and the left external. The explana- 

 tion of such singular correspondences cannot be found in the anato- 

 mical arrangement of the muscles themselves, nor in that of the 

 nervous filaments by which they are directly supplied, but must be 

 looked for in some special endowment of the nervous centres from 

 which they originate. 



EEFLEX ACTION OF THE SPINAL CORD. The spinal cord, as we 

 have thus far examined it, may be regarded simply as a great nerve ; 

 that is, as a bundle of motor and sensitive filaments, connecting 

 the muscles and integument below with the brain above, and 

 assisting, in this capacity, in the production of conscious sensation 

 and voluntary motion. Beside its nervous filaments, however, it 

 contains also a large quantity of gray matter, and is, therefore, 

 itself a ganglionic centre, and capable of independent action as 

 such. We shall now proceed to study it in its second capacity, as 

 a distinct nervous centre. 



If a frog be decapitated, and the body allowed to remain at rest 

 for a few moments, so as to recover from the depressing effects of 

 shock upon the nervous system, it will be found that, although sen- 

 sation and consciousness are destroyed, the power of motion still 

 remains. If the skin of one of the feet be irritated by pinching it 

 with a pair of forceps, the leg is immediately drawn up toward the 



