132 FUNCTIONS OF REFLEX CENTRES. 



In order to obtain series of tracings which can be conveniently compared, 

 introduce into the primary circuit the self-acting key described in II. i. In 

 this way a number of curves may be drawn on the same abscissa, or on 

 parallel abscissae at convenient distances from each other. Having practised 

 one or other of these methods, proceed to make the following observa- 

 tions : 



, (i.) When a succession of ventricular curves are drawn at temperatures 

 varying from 12 to 18, it is found that the duration of the systole is increased 

 by about o"'i for every degree of temperature. 



(2.) When the ventricle is excited by single induction shocks, following each 

 other at about 10" intervals, each curve is observed to exceed its predecessor 

 in amplitude, the augments gradually diminishing from the beginning to the 

 end of the series. 



(3.) In the muscular tissue of the heart, the period of latent stimulation is 

 much longer than in voluntary muscle. Its duration is about o"' 1 5. To measure 

 it, a vertical line must be drawn on the recording surface, indicating the position 

 of the writing point at the moment that the trigger of the cylinder comes into 

 contact with the lever of the self-acting key (see II. i). 



IV. Functions of the Spinal and other Reflex Centres of the Frog. 



1. The preparation to be used in the following experiments is obtained by 

 severing the spinal cord immediately behind the medulla oblongata and intro- 

 ducing, by the opening made for this purpose, a wooden plug into the cranial 

 cavity, so as to destroy its contents. This having been done, it is placed on a 

 sheet of moist filter-paper, resting on its ventral surface with the hind limbs 

 extended, and covered with a bell jar. For a time it remains motionless, but 

 eventually assumes a position which differs but little from that of a living frog. 

 Observe the differences. 



2. Prepare half-a-dozen pieces of filter-paper, each an eighth of an inch 

 square, and some strong acetic acid. Turn the preparation over, and after 

 observing that the natural position is not resumed, apply one of the squares, 

 after moistening it with acetic acid and drawing off excess by touching with 

 dry filter-paper, to the inside of the right thigh, and observe the result. 

 Repeat the experiment, holding the right foot. Next, attach the preparation 

 to a suitable holder in such a way that the trunk may be steadily supported 

 and the limbs may hang freely, and apply the squares in succession to different 

 parts of the surface, as e.g., to the skin on either side of the tendo Achillis, or 

 to either flank. Ol serve in each case that the muscular response which 

 results from excitation of the same part of the surface of the body is always 

 the same. 



3. Arrange a second preparation as last described, using a holder so con- 

 structed that the limbs may be suspended at any desired height above the 

 table. Prepare several beakers of water acidulated respectively with I, 2, 3, 

 4 and 5 per thousand of sulphuric acid, and place some of each mixture in 

 a saucer. Beginning with the weakest of the acid liquids, bring down the 

 preparation with the rack and pinion, until the tip of the longest toe is 



