178 ANIMAL PHYSIOLOGY. 
of air, which, it will be apparent, must be capable of communi- 
cating very slight changes if the apparatus responds readily to 
the alterations in volume of the inclosed air. 
Fig. 175 represents a Marey’s tambour, which consists essen- 
tially of a rigid metallic case provided with an elastic top, to 
which a lever is attached, the whole being brought into com- 
munication with a column of air in an elastic tube. The work- 
ing of such a mechanism will be evident from Figs, 174 and 176. 
Fic. 176.—Tambours of Marey erreneee for transmission of movement (after McKendrick). 
a, receiving tambour ; b, india-rubber tube; c, registering tambour ; d, spiral of wire, 
owing to elasticity of which, when tension is removed from a, the lever ascends. 
The greatest danger in the use of such apparatus is not fric- 
tion but oscillation, so that it is possible that the original move- 
ment may not be expressed alone or simply exaggerated, but 
also complicated by additions, for which the apparatus itself is 
responsible. 
Apparatus of this kind is not usually employed much for 
experiments with muscle; such an arrangement is, however, 
shown in Fig. 174, in which all will be seen—a metronome, the 
pendulum of which, by dipping into cups containing mercury, 
makes the circuit. Such or a simple clock may be utilized for 
indicating the longer intervals of time, as seconds. 
A SINGLE SIMPLE MuscULAR CONTRACTION. 
Experimental Facts —The phases in a single twitch or muscu- 
lar contraction may be studied by means of the pendulum 
