30 NERVOUS SYSTEM. 



stead of producing electricity, there is, on the contrary, 

 negative oscillation (as has been before remarked), that is 

 a weakness or disappearance of the normal current of 

 repose. 



In a nerve displaying activity, there appears to be a sort 

 of molecular vibration which is propagated from point to 

 point at the rate of 28 to 30 metres to the second. This 

 molecular vibration extends both ways along the nerve; 

 when the stimulus is applied midway, its existence is evi- 

 dent only at the nervous extremity, where an organ suitable 

 for its reception may be found ; as, for instance, towards the 

 central end for sensitive nerves, and at the surface or periph- 

 ery for the motor nerves. Thus it may be noticed that 

 the terms centripetal and centrifugal depend upon the dif- 

 ferent connections, and that both can conduct, indifferently, 

 either way (Yulpian). 



4. Excitants of the Nervous System. Those excitants 

 which can set in motion the functions of the nerves are 

 numerous. Some of these are chemical, such as acids, 

 ammonia, &c. ; these agents, it will be seen, excite likewise 

 the muscles, but in this case they need not be so concen- 

 trated as in the former. Others may be in the nature of 

 mechanical or physical excitants ; as, for instance, a blow, 

 electricity or heat. Electricity seems to excite the nerves 

 only by the sudden changes it produces in their molecular 

 condition ; thus a current applied to a nerve affects its ac- 

 tion, only when it begins or terminates its passage through 

 the nerve; during its passage no action is evident. In 

 order to excite nerves, sudden electrical discharges must be 

 applied, and this is the reason for the employment of an 

 induced current, frequently interrupted. At each interrup- 

 tion, there ensues an excitation of the nerve. In normal 

 physiological conditions, the external excitors are brought to 

 bear upon the ends of the so-called sensitive nerves ; certain 

 of the peripheral organs of this class (organs of special 

 sense) exist where particular agents (light, sound, heat, 

 odors, &c.), give rise to special excitations. 



Finally, the central organs act as physiological excitants in 

 the reflex order, where they only transmit previously received 

 excitations, and in the phenomena called voluntary (which 

 are doubtless a more or less complex form of reflex actions). 

 This is due to the power which the nerve globules possess of 

 storing up certain excitations (memory), whose manifesta- 

 tions they allow only at a given time. We may perhaps 



