Physiology of Nerves and Nerve-cells. 507 



insulators, so that insulation of the nerve which prevents its normal 

 current from leaving it is no bar to its influencing or being influenced 

 by electric action in another body if near enough. 



The nerve differs from the muscle in the fact that when it is excited 

 it simply conveys the excitement to the muscle without itself contract- 

 ing. But the muscle when excited contracts, thereby doing work. 

 Each muscle fibre has its corresponding nerve fibre, and it is shown that 

 the ultimate subdivisions of the nerve fibre, called the fibrillae, penetrate 

 among the fibres of muscles, and each muscle fibre finally is reached by 



FIG. 239. Two Muscle Fibres of a mammal, 

 showing their connection with nerve fibres. 



c, d.ThQ Muscle fibres. 



a, 6. The Nerve fibres. 



e. The terminal Nerve Plate. 



The dotted center of the nerve fibre is the 

 axis cylinder, next, outside of which is the 

 medullary sheath, which ends before reach ing 

 the muscle; and outside of all is the Neuri- 

 lemma, which merges into and combines with 

 the tiarcolemma of the muscle. 



one of these fibrillse, the nerve fibres 

 dividing and subdividing so as to 

 make enough to go around. "When 

 near the muscle fibre, the nerve sud- 

 denly becomes thinner, loses the me- 

 dullary sheath, then again thickens, 

 the neurilemma coalesces with the 

 sarcolemma of the muscle fibre, and 

 the axis-cylinder passes directly into 

 a structure which lies within the sar- 

 colemma pouch in immediate contact with the actual muscle substance, 

 and is called the terminal nerve plate." (Fig. 239.) This nerve plate is 

 like a scab on the bark of the muscle fibre of mammals. In other ani- 

 mals it is different. In the frog, the nerve fibres penetrate into the sar- 

 colemma, where they form a network, bringing their materials into di- 

 rect contact with the matter of the muscle. The nerve fibres, insulated 

 as they are by the neurilemma, pass among the muscle fibres without 

 appearing to affect any of them except the ones to which they are di- 

 rectly connected by the nerve plate, or otherwise. 



The poison curare ( or wourali ), which is obtained from the bark of 

 a species of convolvulus, is used by the Indians of South America for 

 poisoning their arrows. Its effect is to prevent muscular contraction. 

 It is shown that neither the power of the nerve trunks to convey stimu- 

 lus, nor the power of the muscles to contract when stimulated, is in- 

 jured by this poison. The injury, therefore, consists in depriving the 

 nerve of the power of delivering the stimulus to the muscle. This is 

 doubtless due to the fact that near the point where the nerve fibre ex- 

 pands into the plate or "scab, "above described, it is divested of its 



FIG. 239. 



