596 NERVOUS SYSTEM. 



which is reflected back through the motor filaments of the same nerve, and the ordinary 

 reflex movements are observed in the posterior extremities. This is to be expected, inas- 

 much as the posterior extremities have been removed from the influence of the poison. 

 If the anterior extremities, which are completely under the influence of the poison, be now 

 irritated, no movements are observed in these parts, but they take place, as before, in 

 the posterior extremities. The mechanism of this action is easily understood. Reflex 

 phenomena, consisting in the movements of muscles, may be manifested throughout the 

 entire system, following irritation of a single part. An impression made upon the surface 

 is conveyed to the spinal cord, and, if this be sufficiently powerful, motor stimulus may 

 be sent through all of the anterior roots coming from the cord. The impression made 

 upon the anterior, or poisoned extremities, is conveyed by the sensory filaments to the 

 cord and is transmitted to the posterior extremities through their motor nerves, which 

 are intact. The fact of the transmission of the impression from the anterior extremities 

 to the cord shows that the poison does not affect the sensory system. 



In the same way that the woorara-poison paralyzes the motor nerves, leaving the 

 sensory system intact, other agents, as anesthetics, will abolish the sensibility of the 

 nerves without affecting the motor filaments. 



As we have already intimated in another connection, the nerves soon lose their irrita- 

 bility after they have been separated from the centres. This loss of conducting power 

 is attended with important structural changes in the nerve fibres. The tubes lose their 

 normal appearance, and the medullary matter becomes opaque and coagulates in large 

 drops. The axis-cylinder is not so much modified in structure, but it certainly loses 

 its characteristic physiological properties. 



The excitability of the motor nerves disappears in about four days after resection. 

 Of course, in experiments upon this point, it is necessary to excise a portion of the 

 nerve to prevent reunion of the divided extremities; but, when this is done, after the 

 fourth day, galvanization of the nerve will produce no contraction in the muscles, 

 although the latter retain their contractility, as may be shown by the application of direct 

 irritation. This loss of irritability is gradual, and it continues, whether the nerve be 

 exposed and stimulated from time to time or be left to itself; and the loss of excitability 

 progresses from the centres to the periphery. In the researches of Longet upon this sub- 

 ject, it was found that the lower portion of the peduncles of the brain lost their irrita- 

 bility first, then the anterior columns of the cord, then the motor roots of the nerves, 

 and, last of all, the branches of the nerves near their termination in the muscles. 



The sensibility of the sensory nerves disappears from the periphery to the centres, as 

 is shown in dying animals and in experiments with anesthetics. The sensibility is lost, 

 first in the terminal branches of the nerves, next in the trunks and in the posterior roots 

 of the spinal nerves, and so on to the centres. "We have often illustrated this fact in 

 experiments upon the roots of the spinal nerves and in section of the large root of the 

 fifth pair within the cranial cavity. When an animal is brought so completely under 

 the influence of ether that the operation of opening the spinal canal may be performed 

 without inflicting the slightest pain, the posterior roots will be found to be distinctly 

 sensible. We have lately been in the habit, in class-demonstrations, of dividing the fifth 

 pair in the cranium without using an anaesthetic, as the operation is instantaneous and 

 the effects are much more striking than when the animal has been rendered insensible 

 and is allowed to recover ; but, when we have used an anaesthetic, we could never push 

 the effects sufficiently to abolish the sensibility of the root of the nerve. In an animal 

 brought so fully under the influence of ether that the conjunctiva, supplied with branches 

 of the fifth, had become absolutely insensible, the instant the instrument touched the 

 root of the nerve in the cranium, there were evidences of acute pain. Nothing could 

 more strikingly illustrate the mode of disappearance of the sensibility of the nerves 

 from the periphery to the centres. 



The nervous irritability may be momentarily destroyed by severe shock in killing an 



