PHYSIOLOGY OF ATTENTION AND VOLITION. 229 



arteries that they turn on or shut off the blood-current as may seem 

 to be necessary. While not disposed to question their great impor- 

 tance, a good deal might be said in favor of the notion that the mo- 

 lecular agitation in the tissue itself has a direct influence not only in 

 assisting movement by lessening friction, but by exerting positive 

 energy in urging the current onward.* 



Into the controversy on these points, however, I can not enter 

 here. For our present purpose it will be sufficient if the intimate 

 relations between demand and supply be admitted as a matter of 

 fact ; if we can assume that functional activity involves a fuller vol- 

 ume and more rapid movement of blood in the capillaries of a part 

 than does functional rest. 



The last postulate I have to submit is the one on which my sub- 

 sequent argument must mainly rest ; but, unfortunately, it is the one 

 whose soundness is most likely to be questioned. It is, that the mass 

 of blood within the cranial cavity can be neither increased nor dimin- 

 ished directly, nor, indeed, to an appreciable extent within short pe- 

 riods of time. 



A general statement of the argument in its support may lie in a 

 nutshell. The available cubic space within the skull being a fixed 

 quantity, the bulk of its contents must also continue uniform. These 

 contents being the brain-tissue, the blood and the cerebro-spinal fluid, 

 no one of these can be altered without an inverse chancre in one or 

 both of the other contents. Thus, if a degenerative nutrition cause 

 wasting of the brain-tissue, we must have an increase in one or both 

 of the fluid contents, and thus evidence will be got of extreme con- 

 gestion, or of serous effusion, or both. For any such change, how- 

 ever, time is required. Again, no amount of general depletion can 

 reduce the intra-cranial circulation until time is afforded to allow effu- 

 sion of serum to occur, because no mechanism exists for immediately 

 raising any fluid from the spinal canal. Neither, on the other hand, 

 can any increased force of the heart's action make the intra-cranial 

 vessels fuller, for the cerebro-spinal fluid can not be immediately dis- 



* In recent works on physiology it has been the tendency to ignore, if not altogether 

 to deny, the active influence of local molecular change on the capillary circulation. In 

 the higher animals, however, we have what I would consider crucial evidence in favor of 

 its existence. I allude to the portal circulation. Here we have a large mass of blood re- 

 turned from the chylopoietic viscera, which, before it can reach the heart, has to traverse 

 the ramifications of the portal system of vessels. It will be at once admitted that con- 

 siderable force must be required for the purpose. Now, if a wis a terr/o alone be employed 

 in moving the blood onward, the whole stress so occasioned must in this instance be borne 

 by the mesenteric veins. The backward pressure within these vessels will be as great as 

 what is required to transmit the blood onward to the vena cava. Is it in the least 

 probable that the thin walls of these veins could bear such a strain? I rather think that 

 we have here evidence that, while the general circulation may be sustained by the action 

 of the heart, certain forces acting at the capillaries give indispensable aid in transmitting 

 the blood through the latter. 



