714 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



In the second case the rate of transmission of the nerve impulse has 

 been placed about twenty-eight meters per second. After the latent 

 period has been completed the muscle then commences to shorten, at first 

 slowly and then more rapidly, and then again more slowly until the 

 maximum shortening is reached, the duration of active contraction occu- 

 pying about the one-twentieth part of a second. As soon as the maxi- 



FIG. 279. DIAGRAMMATIC CUKVES ILLUSTRATING THE MEASUREMENT OF THE 



VELOCITY OF A NERVOUS IMPULSE. (Foster.) 



(To be read from left to right.) 



The same nerve-muscle preparation is stimulated (1) as far as possible from the muscle, i'2) as near as 

 possible to the muscle, both contractions being registered in the same manner on the pendulum myograph 



In (1) the stimulus enters the nerve at the time indicated by the line, a ; the contraction, shown by the 

 dotted line, begins at b> ; the whole latent period is, therefore, indicated by the distance from a to bl. 



In (2) the stimulus also enters the nerve at a, the contraction begins at b, and is shown in the un- 

 broken line ; the latent period, therefore, is indicated by the distance from a to b. The time taken up in 

 the passage of the nerve impulse in the length of nerve between 1 and 2 is indicated by the distance be- 

 tween b and bt, and may be measured by the tuning-fork curve below. The distance between these 

 curves is exaggerated for the sake of simplicity, no value being given for the rate of vibration of the 

 tuning-fork. 



mum contraction is reached relaxation commences, following the same 

 general course as in shortening, relaxing first slowly then more rapidty, 

 and then more slowly again, the general duration of the active relaxation 

 being somewhat longer than that of contraction. Such are the general 

 characteristics of the curve of a single muscular contraction produced 

 by a single stimulus. 



If a single stimulus be allowed to follow the first it will be followed, 



FIG. 280. TRACING OF A DOUBLE MUSCLE CURVE. 



(To be read from left to right.) 



(Foster.) 



While the muscle was engaged in the first contraction (whose complete course, had nothing intervened. 

 is indicated by the lower line in the muscle-curve) a second induction shock was thrown in at such a time 

 that the second contraction began just as the first was beginning to decline. The second curve is seen to 

 start from the first, as does the first from the base line. 



like the first, by a single muscular contraction. If the interval between 

 the second and first muscular contraction be gradually reduced, a point 

 will ultimately be reached in which the second stimulus enters the nerve 

 before the contraction produced by the first has passed off. The result 

 will be that the muscle will undergo a second shortening, and such a 



