220 SPECIAL PHYSIOLOGY. 



and finally disappears, the nerve itself becoming converted into cellu- 

 lar and fatty tissue ; the nearer the point of division to the nervous 

 centre, the quicker these changes occur. Sensory nerve-fibres, when 

 divided, undergo degeneration, both in their central and peripheral 

 portions; in the former, because they can no longer conduct impres- 

 sions, and in the latter, on account of their separation from the cen- 

 tral organ. Excitability is thus shown to be an inherent property of 

 nerve, but requires, for its permanent maintenance, a connection with 

 a nervous centre. It is destroyed by mechanical injuries, by chemical 

 changes, and by very strong electrical shocks. Moderate stimulation 

 increases the excitability ; stronger stimuli weaken or destroy it. 

 When exhausted in regard to weaker stimuli, it may still be called into 

 play by more powerful or by other kinds of stimuli ; a succession of 

 different stimuli is not so exhaustive as the continuance of the same 

 stimulus. 



The sudden application of any stimulus, is one condition of its 

 action ; for the mechanical stimulus of pressure gradually applied to 

 a motor nerve, and increased in intensity, produces no convulsions, 

 even if the nerve be ultimately destroyed. Sudden mechanical shocks 

 alone produce muscular contractions. Again, a ligature slowly tight- 

 ened around a sensory nerve, causes merely a numbness, and at length 

 total insensibility ; whereas, if it be suddenly tied, intense pain is 

 produced. The electrical current has been supposed to act, by causing 

 mere mechanical disturbance in the particles of the nerve-fibres ; but 

 this view is inconsistent with the known changes in the electrical state 

 of nerves, when acted upon by galvanic currents, or by mechanical, 

 or other, stimuli. 



The action of particular stimuli deserves notice. Thus, the influence 

 of chemical stimuli upon nerves is slow r probably on account of the 

 resistance offered by the neurilemma or sheath ; they are said to act 

 more readily on sensory than on motor fibres, the former of which are, 

 moreover, acted upon by a greater variety of chemical stimuli. Strong 

 solutions of ammonia, and alcohol, powerfully stimulate the motor 

 nerves ; so also do solutions of nitrate of potash and hydrochloric acid, 

 and even very weak solutions of soda or potash. Certain powerful 

 agents, such as bisulphide of carbon and strong mineral acids, destroy 

 the nerve so quickly that no convulsions ensue. On injecting water into 

 the vessels of a muscle, strong contractions take place, due, it is sup- 

 posed, to the effect of the fluid on the fine terminations of the nerves 

 (Schiff); but if water be applied to the trunks of the nerves, no move- 

 ments occur. The gradual abstraction of water from a nerve, is not 

 followed by muscular contractions ; but if rapidly effected, tetanic 

 spasms are produced. Certain poisons are found to lessen or destroy 

 the nervous excitability, acting more or less suddenly in different 

 cases. Some of these, as narcotics, for example, if applied locally to 

 a nerve, will deaden its excitability at the point of application only ; 

 but, if introduced into the blood, they operate generally. The nerves 

 are never the channels by which poisons are conducted into the sys- 

 tem. However the sensory nerves are affected by heat or cold, it is 

 only changes of temperature that are recognized by the sensorium, as 



