30 MUSCULAR WORK 
with the system a delicate petroleum manometer and admitting oxygen be- 
fore and after the experiment until a certain definite tension upon the rubber 
diaphragm is indicated. 
The rate of ventilation commonly used in resting respiration experiments 
in this laboratory is about 35 liters per minute, but in this research it was 
found necessary, owing to the excessive ventilation of the lungs and deep 
inspiration during severe muscular work, to increase the rate of ventilation 
to 85 liters per minute. Under these conditions the air-current passed by the 
mouthpiece so rapidly that at no time could there be any rebreathing of the 
air, since the rate of flow was always greater than the intake of the lungs. 
During the rest experiments the rate of ventilation was reduced to 35 liters 
per minute. 
The subject who was used in most of the experiments reported preferred 
to use the mouthpiece rather than the nosepiece. Indeed, the amount of 
ventilation permitted with any form of nosepiece would be too small for the 
intense muscular work done in these experiments. It was found, furthermore, 
that when riding on the bicycle at high rates of speed, there was an uncontrol- 
lable lateral head motion which made it difficult to retain the mouthpiece 
when fastened directly to the rigid brass tube on the valve providing the 
opening into the air-circuit; consequently it was necessary to arrange for a 
somewhat longer and more flexible connection. The device used in the mus- 
cular-work experiments is shown in fig. 3, which gives the details of the 
connection between the subject and the air-circuit. 
With the ordinary form of connection, which was used in some of the 
rest experiments, the subject breathed into and out of the main air-circuit 
through the valve n, but for the muscular-work experiments and all the later 
rest experiments a short length of rubber tubing k was attached to the valve- 
piece m, to which was attached a brass tee n and the mouthpiece g. The 
flexible tube k allowed considerable lateral motion of the mouthpiece g when 
the subject was riding vigorously, which was impossible with the ordinary 
connection, but on the other hand, the dead air-space was increased. Experi- 
ments subsequently made by Mr. T. M. Carpenter of this laboratory have 
shown that so short a dead air-space as is represented by n and k is absolutely 
without effect upon the metabolism of resting man. As we did not have the 
results of Mr. Carpenter's important observations at the time of the research, 
it was considered necessary to make some provision so that this dead air- 
space could have no possible effect upon the respiration during severe muscular 
work. This was done by attaching a supplementary piece of rubber tubing s 
to the 3-way valve r, and connecting it with the brass tee n. During the 
experimental period, the 3-way valve r was turned so that the air passing 
through the pipe p was deflected and entered the rubber tube s leading to the 
mouthpiece g. As the air passed through the tube s and directly by the open- 
ing of the mouthpiece g, it was inspired by the subject and the air expired 
entered the current which flowed through the tubing k and the valve m to the 
main air-circuit. In this way the dead air-space between the subject and the 
ventilating current was reduced to a minimum. Furthermore, this arrange- 
ment facilitated easy respiration. 
