MOVEMENTS OE RESPIRATION ON THE CIRCULATION OE THE BLOOD. 575 
gutta percha, one arm of which is connected with the trachea, the other with the elastic 
bag by a flexible tube of equal width with itself. The third arm remains open. The 
slight resistance afforded by this arrangement to the flow of air produces sufficient 
movements of the lever to indicate the exact duration and relative intensity of the 
respiratory movements. 
b. Variations of arterial pressure . — As the purpose of the investigation was not to 
measure the absolute arterial pressure, but to determine the variations of pressure with 
reference to their duration and order of succession, it appeared unnecessary to employ a 
more complicated apparatus than the following, from the construction of which it will 
be seen that the results obtained by it are subject to the error due to the difference of 
level between the arterial aperture of the animal and the surface of the mercury at the 
commencement of the observation. The error in question would rarely exceed a tenth 
of an inch of mercury. The haemadynamometer employed is a U-shaped tube of glass, 
of which the longer arm measures 15 inches, the shorter 10 inches. It differs from the 
haemadynamometer of Poiseuille in this respect, that the attached arm, which is the 
longer of the two, is of smaller calibre than the open arm, the area of the mercurial 
column contained in the latter being about twelve times as great as that in the other. 
Hence for every variation of an inch of pressure the surface of the mercury in the open 
tube moves only one-thirteenth of an inch. The movements of the mercury are trans- 
mitted to the recording cylinder, by a lever of the same length and supported on a 
moveable bearing in the same manner as the one described above. This lever is con- 
nected by a vertical rod of iron wire with a conical cork float which rests on the surface 
of the mercury contained in the wide arm, its base being concave, so as to correspond 
with the convexity of the meniscus. The joint by which the vertical rod is connected 
with the lever is so arranged that for every inch of variation in pressure a nearly vertical 
movement of the extremity of the lever carrying the brush, amounting to three-tenths 
of an inch, takes place. In order that the lever may accurately follow all the oscillations 
of the mercury, its dead weight is nearly neutralized by a weight of lead suspended 
beyond the fulcrum. A similar counterpoise is attached to the lever for recording the 
respiratory movements. 
c. The recording apparatus consists of two cylinders, each of 10 inches in diameter, 
both of which revolve vertically. They are connected together by the band of paper on 
which the tracing is to be made ; this band as it is delivered from the one is wound 
round the other. The motion of the receiving-cylinder is produced by the descent of a 
weight and regulated by clockwork to which the feeding-cylinder is adapted. The 
movements of the levers are inscribed on the paper in its transit from one cylinder to 
the other. The rate of movement of the paper admits of being varied according to 
the requirements of the experiment, but in most instances it was such that 10 inches 
were delivered in a minute. It is obvious that although the greatest care may be taken 
to adjust the levers in such a manner at the commencement of each observation that the 
points of the brushes are in the same vertical line, this relation can only be maintained 
