Page 20. 
20 CARDIOGRAPH TRACINGS 
get merged into one curved line. The subsequent fall (d) is here 
seen to have become much diminished, and the next rise and fall (e) 
of greater duration. 
In the slow pulses (Figs. VI, [X) the fall after the small rise pre- 
ceding the main ascent (a) is inconsiderable, or nil, which makes that 
rise appear as part of the main one, which is not the case. 
The rise c has now become more marked, while d has diminished 
so much that it is no longer the highest point of the trace, that now 
being at the end of the rise preceding the main descent (f), which 
is frequently found to be double. 
It is to be noticed that as the pulse gets slower, the generally 
ascending line between & and a gets longer; also that at all rates 
there is a great similarity in shape in the fall and rise between the 
points and &, which is quite characteristic of that part of the curve. 
A precise knowledge of the causes of these various changes in the 
direction of the human apex trace will always be somewhat deficient, 
from the impossibility of vivisectional verification, and from the fact 
that the relation of the organs concerned is different in man to what 
it is in animals, from which, otherwise, arguments from homology 
might have been more extensively employed. 
By means of synchronous traces from the exterior and interior of 
the heart of the horse Marey explains his apex trace, which in the 
main resembles that from the human subject. He shows that the rise, 
here called a, results from the contraction of the auricles, which makes 
it clear that that event occurs much nearer to the commencing ven- 
tricular contraction, represented by the origin of the main ascent, than 
is supposed by many. He also shows that the semilunar valves close 
at the break, single in his trace, in the main descent. The irre- 
gularities in the systolic interval he considers due to vibration of the 
blood caused by the tightening of the auriculo-ventricular valves, but 
his results were recorded after having been communicated to india- 
rubber tubes filled with air, and the undulations probably originating 
in them. 
It is necessary in attempting to explain these traces, especially 
when comparing different rapidities of pulse, always to bear in mind 
Marey’s most important law, that “the arterial tension (blood poten- 
tial) varies inversely as the rate of the pulse.”” This law, though dis- 
puted by some, must closely approximate to the truth, because by it 
so many facts with regard to the circulation of the blood are perfectly 
explained, that cannot be in the least accounted for elsewise; and it 
will shortly appear how auch it assists in interpreting the curves 
under consideration. 
After the auricles have contracted at a, the commencing ventri- 
cular action originates the main rise, which continues uninterrupted 
