INDEPENDENCE OF NERVOUS FILAMENTS. 407 



ward, that of the sensitive filaments from without inward. Immedi- 

 ately after the separation of the frog's leg from the body, irritation 

 of the nerve at any point produces muscular contraction in the 

 limb below. As time elapses, however, and the irritability of the 

 nerve diminishes, the galvanic current, in order to produce con- 

 traction, must be applied at a point nearer its termination. Subse- 

 quently, the irritability of the nerve is entirely lost in its upper 

 portions, but is retained in the parts situated lower down, from 

 which also, in turn, it afterward disappears ; receding in this man- 

 ner farther and farther toward the terminal distribution of the 

 nerve, where it finally disappears altogether. 



On the other hand, sensibility disappears, at the time of death, 

 first in the extremities. From them the numbness gradually creeps 

 upward, invading successively the middle and upper portions of the 

 limbs, and the more distant portions of the trunk. The central 

 parts are the last to become insensible. 



3. Each nervous filament acts inde2iendently of the rest throughout its 

 entire length, and does not communicate its irritation to those which are 

 i ' n proximity with it. It is evident that this is true with regard to 

 the nerves of sensation, from the fact that if the integument be 

 touched with the point of a needle, the sensation is referred to that 

 spot alone. Since the nervous filaments coming from it and the 

 adjacent parts are all bound together in parallel bundles, to form 

 the trunk of the nerve, if any irritation were communicated from 

 one sensitive filament to another, the sensation produced would be 

 indefinite and diffused, whereas it is really confined to the spot irri- 

 tated. If a frog's leg, furthermore, be prepared, with the sciatic 

 nerve attached, a few of the fibres separated laterally from the 

 nervous trunk for a portion of its length, and the poles of a galvanic 

 battery applied to the separated portion, the contractions which 

 follow in the leg will not be general, but will be confined to those 

 muscles in which the galvanized nervous fibres especially have 

 their distribution. There are also various instances, in the body, 

 of antagonistic muscles, which must act independently of each 

 other, but which are supplied with nerves from a common trunk. 

 The superior and inferior straight muscles of the eyeball, for 

 example, are both supplied by the motor oculi communis nerve. 

 Extensor and flexor muscles, as, for example, those of the fingers, 

 are often supplied by the same nerve, and yet act alternately with- 

 out mutual interference. It is easv to see that if this were not the 



