1833.] Variations of Latency in certain Skeletal Muscles. 285 



the most rapid movement of which the animal is capable. The semi- 

 membranosus passes over three joints, and a considerable extent of 

 contraction is necessary in order to enable it to flex the foot on the 

 knee, and the thigh on the pelvis. Both of these muscles arrive 

 speedily at a maximum of contraction, *18" and *24", and coincidently 

 with this rapid action we have the relatively short latency quoted. 

 There is a considerable difficulty in estimating the length of curve of 

 the tortoise muscle, owing to its gradual return to the base line. 

 Usually a notch in the descending part of the curve shows the 

 distinction between the active curve and the after shortening which is 

 so marked a feature in the action of the muscles of this animal ; but 

 occasionally the one passes into the other by insensible gradations. 

 We have, however, endeavoured by varying the burden to produce 

 some indication of this notch when none at first existed, and we have 

 confined ourselves to the results which we believe to be correct. 



Warm-blooded Animals. 



The muscles of warm-blooded animals were examined in situ, the 

 circulation being as little as possible interfered with, and the animal 

 kept well under the influence of anaesthetics. A modification of 

 Ranvier's rabbit-holder was utilized in order to support the animal in 

 the necessary relationship to the recording lever. The head-holder at 

 one end of a small board, consisted of a loop of metal covered with 

 cord, which received the snout or beak and anterior part of the head, 

 whilst a piece of string run through two openings in the loop, and 

 behind the occiput kept the head in position, and facilitated the 

 administration of ether. The board which served to support the 

 animal was perforated with numerous holes, through which strings 

 attached to the animal passed. The under surface of the board was 

 fitted with a ball and socket joint, which could be clamped firmly to 

 the sliding table on which the lever rested. By a screw which fixed 

 the cup around the ball, this joint could be tightened when the board 

 was adjusted to any given position, and thus the muscle to be 

 examined could be brought into direct line of traction with the lever 

 without altering the attachment of the animal or otherwise disturbing 

 the apparatus. The axis of the lever was provided with a short arm 

 projecting on the side opposite to the lever, to which a thread passing 

 from the muscle was attached. The arm was drawn downwards by 

 the contracting muscle, and the lever therefore upwards, so that a 

 curve in every way comparable to those obtained by direct traction was 

 drawn. 



Mammals. 

 Table IV.— Adult Rat. 

 The gastrocnemius of the rat draws a curve with a double summit. 



