DEPENDENCE OF NERVOUS POWER ON SUPPLY OF BLOOD. 233 



materials, and requiring a strengthening and even a stimulating regimen. 

 The skill of the practitioner is often put to the test, in the due 

 discrimination between these states. 



402. The preceding examples mark the influence of various causes 

 upon the actions of the vesicular matter of the brain ; others might be 

 adduced to show, that the vesicular substance of the spinal cord is also 

 liable to have its powers depressed or excited ; but these will be best 

 adverted to hereafter, when the distinct functions of that organ are 

 under consideration (CHAP, xn.) We may simply notice, that the stimu- 

 lating effect of Strychnia is peculiarly and most remarkably exerted 

 upon the vesicular substance of the spinal cord ; and that a correspond- 

 ing state, in which violent convulsive actions are excited by the most 

 trifling causes, sometimes presents itself as a peculiar form of disease, 

 named Tetanus, which may be either idiopathic, depending probably 

 upon a disordered condition of the blood, or traumatic, consequent upon 

 the irritation of a wound. 



403. But, as formerly remarked, it is not in the Nervous centres 

 only, that changes originate. Whenever an impression is made upon 

 the surface of the body, or upon the organs of special sense, which, 

 being conducted to the nervous centres, either excites a sensation in the 

 brain ( 390), or a reflex action through the spinal cord ( 392), the 

 reception and propagation of such impression at the extremities of the 

 sensory nerves requires a set of conditions of the same kind with those, 

 which we have seen to exist in the nervous centres. In fact, if we re- 

 gard the course of the motor nerves as commencing in the nervous 

 centres and terminating in the muscles, we may with equal justice con- 

 sider that of the sensory nerves as originating in their peripheral extremi- 

 ties, and terminating in the sensorium. And, as already stated ( 381), 

 precisely the same kind of vesicular structure exists in some (probably 

 in all) of the peripheral expansions of the sensory nerves, as makes up 

 the gray substance of the brain and spinal cord. Now it is easily shown, 

 that the circulation of the blood through these parts is just as necessary 

 for the original reception of the impressions, as is the circulation through 

 the brain to their reception as sensations, and to the origination of 

 motor impulses by an act of the will. We find that anything which 

 retards the circulation through a part supplied by sensory nerves, 

 diminishes its sensibility ; and that if the flow of blood be completely 

 stagnated, entire insensibility is the result. A familiar example of this 

 is seen in the effects of prolonged cold ; which, by diminishing, and 

 then entirely checking, the flow of blood through the skin, produces 

 first numbness, and then complete insensibility of the part. This result, 

 however, may be partly due to the direct influence of the cold upon the 

 nerve-vesicles themselves ; depressing their peculiar vital powers ( 97). 

 The same effect is produced, however, when the supply of blood is 

 checked in any other way ; as, for example, by pressure on the artery, 

 or by obstruction in its interior. Thus when the main artery of a limb 

 is tied, numbness of the extremities is immediately perceived ; and this 

 continues, until the circulation is re-established by the collateral branches, 

 when the usual amount of sensibility is restored. Again, in the gan- 

 grene which depends upon obstruction of the arterial trunks by a 



