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EXPERIMENTAL PII YSIOLOG Y 



the lungs can be modified in two ways. First by varying the compression of 

 the tube at m. The increase in resistance at m raises the pressure at n, and the 

 lungs are therefore more distended. The second method is to vary the 

 pressure of the blast of air. This is attained by compressing c, an increased 

 resistance to the outflow of water leading to a higher pressure of the air. The 

 screw clips are so arranged that sufficient distension is obtained with the 

 lowest pressure of air. Expiration is brought about by the elastic c ontraction 

 of the lung which drives out the air through the tube M. 



The pericardium is now slit up and its cut edges stitched to the 

 thoracic wall on either side. The heart is thus exposed and is attached 

 to the recording apparatus, which consists of two levers, L 1 and l' 2 , 

 fig. 110, which are moved by two fine cotton threads passing over two 



Fig. 110. — Arrangement of Levers for Recording the Movements of the 

 Mammalian Heart by Attaching Threads to the Auricle and Ventricle 

 respectively. 



pulleys, P 1 and p 2 . Each thread terminates in a sharp hook, which 

 is passed into a small piece of the ventricular and auricular walls 

 respectively. If the thread from the auricle does not pass freely over 

 the surface of the ventricle it may be made to glide over a glass rod 

 held transversely above the heart, so that the thread is quite free from 

 surrounding parts. The magnification of the lever l 1 for the ventricle 

 is 3-fold ; of the lever l 2 for the auricle 4-fold. Each lever is loaded 

 by weights, w 1 , w 2 , placed near its axis, so as to avoid effects of inertia 

 as far as possible. 



The writing levers are brought to the horizontal position by 



