352 OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



And in states of peculiar functional activity of the nervous centres (such as that 

 produced in the spinal cord by the administration of strychnine), it would appear 

 as if motor impulses originate spontaneously, just like certain muscular contrac- 

 tions ( 328); how far this must be accounted a normal modus operandi, how- 

 ever, is yet uncertain. 



354. The influence of excitants, repeatedly and powerfully applied to the 

 Nervous tissue, is undoubtedly (as in the case of Muscle) to weaken, and at last 

 to exhaust, its power of responding to them ; this is seen alike in experimental 

 researches, and in the ordinary course of vital action ( 360). The excitability 

 thus exhausted, can only be regained by an interval of repose, during which the 

 nutritive operations may restore the tissue to its pristine integrity, and thus 

 prepare it for renewed activity. On the other hand, the moderate and regular 

 excitation of Nervous activity, with due intervals of repose, is favorable to its 

 nutrition ; and this is especially to be noticed in the case of the Brain, an in- 

 creased development of which, especially in young subjects, is continually to be 

 noticed as consequent upon the continuance of a state of high functional activity. 

 It seems probable that this augmentation is principally due, as in the case of 

 Muscle, to the increased afflux of blood to the organ, which its functional activity 

 induces. That the interruption of that activity not only suspends nutrition, but 

 rapidly induces degenerative change, has been already shown ( 347). 



355. Of the condition on which the functional activity of the Nervous system 

 is dependent, the first H, of course, the integrity of its own structure ; thus, an 

 interruption in any part of the "nervous circle" will prevent the manifestation 

 of its peculiar endowments. But, however perfect and complete its condition, 

 no action can take place in it without a supply of oxygenated blood ; which is 

 more immediately necessary for the maintenance of the Nervous power, than it 

 is for that of any other tissue whatever. That this supply is not so essential, 

 however, to the conduction of impressions, as it is to their reception and reflexion, 

 would appear alike from the difference between the amounts of blood transmitted 

 to the trunks of the nerves in their course, and to their peripheral and central 

 terminations ; and from the different effects of the suspension of the circulation 

 upon each. For the nerve-trunks are not peculiarly vascular, and retain 

 their power of transmission for some time after the movement of the blood has 

 ceased. On the other hand, both the nervous centres and the organs of sense 

 receive an enormous supply of blood; and the effects of its interruption are imme- 

 diately manifested in the most striking manner. Thus, if the circulation through 

 the Brain be suspended but for an instant, insensibility and loss of voluntary 

 power supervene, and continue until it is restored; as was shown in the follow- 

 ing experiment by Sir A. Cooper. After having tied both carotid arteries in a 

 dog, he compressed the Vertebral trunks ; and immediate insensibility came on, 

 the animal at the same time falling powerless. But convulsive movements oc- 

 curred at the same time ; showing that the functions of the spinal cord were 

 not suspended, but only deranged. As soon as the blood was readmitted to 

 the brain, the animal recovered it consciousness and voluntary power, and stood 

 on its legs again ; the convulsive movements ceasing at the same time. 1 In 

 Syncope, moreover, the circulation through the Spinal cord is weakened, by the 

 failure of the heart's action, to the same extent as the flow of blood through the 

 Brain ; and a general cessation, not merely of muscular movement, but of all 

 power of exciting it, is the immediate result. No sooner, however, is the circu- 

 lation fully re-established, than the power of the Nervous centres is restored. 

 Again, the influence of diminished circulation at the origins of the afferent nerves, 

 is shown in the deficient impressibility of these nerves at the part affected. Thus, 

 if the movement of blood through the capillaries of a limb be stagnated (whether 



" ' Guy's Hospital Reports," vol. i. 



