23 2 THE POPULAR SCIENCE MONTHLY. 



the direction of activity. The molecular agitation occasions a neces- 

 sity and an attraction for more blood, and determination of this takes 

 place all the more freely on account of the quiescence of the larger 

 part of the brain. The latter has, as it were, loosened its hold on the 

 circulation, and the impetus toward those parts which have an attrac- 

 tion for it is thus all the stronger. The increased activity of the cir- 

 culation then reacts on the energies of the tissue, and the mental effect 

 produced is therefore greater. 



If, now, we turn the picture, we find the lights and shadows have 

 changed places. Let the mind be intent on solving some problem, or 

 be engaged on some work requiring close attention and nicety of 

 handling, and the impression which formerly so completely took pos- 

 session of the consciousness may now not in the least be felt. Here, 

 too, physiological conditions are at work. The impression fails, not 

 simply because the consciousness is otherwise engaged, but because 

 the track along which it is to travel is not now in a fit condition for 

 responding to the stimulus. It is out of focus. The momentum of 

 the circulation is now directed toward the centers of ideation and 

 voluntary motion, and this implies a derivation from, and consequent 

 weakening of, functional vigor in the sensory ganglia. 



If the above reasoning be legitimate— even approximately so — it 

 becomes a matter of detail to apply the principle in other directions. 

 In speculating on any point in mental physiology, we have something 

 more than the molecular action of the brain itself to consider. The 

 capillary circulation, too, has its laws, and the encephalic circulation 

 its peculiarities, and a certain balance in the latter must be maintained 

 if cerebration is to be healthy and its outcome exact. 



In perfectly normal action it is likely that the molecular changes 

 are the dominant factor and keep the circulation under control ; but 

 not unfrequently the mass and velocity of the circulating fluid may 

 determine the sphere as well as the character of the activity, and thus 

 have effect on the outcome, whether muscular or mental. 



If the cells of one center or class of centers be too readily explosive, 

 they may attract the blood so strongly as to inhibit the function of 

 other parts of the brain by the comparatively anajmic condition these 

 are thus left in. Of this we have an illustration in the phenomena of 

 an epileptic seizure. Here we have the blood determined in such vol- 

 ume to the motor centers that those which are more immediately re- 

 lated to consciousness have not sufficient left to enable tbem to sustain 

 function with. Some writci's seem to insist that during the seizure 

 the whole brain becomes almost bloodless ; but it would be as philo- 

 sophical to expect a water-wheel to revolve violently by its supply of 

 water being cut off as that the energy of the brain can be prodigally 

 expended in defiance of ordinary physiological conditions. 



On the other hand, if the attractive power of some center is under 

 the normal, this may allow a determination to other centers to be 



