648 
PHYSIOLOGY 
Table I. — Anatomical, chemical and electric characteristics of ventricular muscle in man and some experimental 
animals 
Ref. 
Man 
Dog 
Cat 
Cow 
Pie 
Guinea 
Pig 
Rabbit 
Rat 
Chicken 
Frog 
J? 10€1^ Ql&I11€t€l* (/i) - - ,, 
1, 2 
16 
16 
16 
10 
10 
13 
11 
9 
6 
5 
++ 
++ 
++ 
++ 
++ 
++ 
++ 
+ 
— 
— 
T^nVl^lMTQ T^^Tl a+>«»a t-JrtTl 
99 
bUyo 
60% 
100% 
95% 
7 
7 
100% 
— 
A 19 9Q 9Q 
18 
18 
18 
11 
14 
12 
12 
20 
it 19 
0.31 
0.28 
0.49 
0.29 
0.40 
0.39 
0.25 
Ca muscle* (mg%) 
12, 23-26 
4.6 
5.0 
5.0 
2.5 
6.6 
6.8 
6.7 
13.0 
3.0 
Resting pot. (mV). 
1, 2-5, 28 
29, 7, 8-10, 25 
86 
85 
88 
85 
79 
83 
81 
83 
64 
84 
Action pot. (mV). 
115 
105 
116 
110 
106 
105 
112 
101 
89 
102 
Spike 
+ 
4- 
++ 
Plateau 
+ 
+ 
+ 
++ 
++ 
+ 
+ 
+- 
+ 
++ 
Conduct. Vel. (m/sec) 
1 
1.3 
1.0 
.96 
0.17 
Q-aTc 
6 
100% 
90% 
95% 
130% 
120% 
115% 
115% 
25% 
100% 
1507o 
Chronaxie (msec) 
13, 16-20 
0.3** 
0.2*** 
2.0 
* In fresh fat-free and blood-free muscle. 
** In vitro value = 3 ms.i" The extreme in vivo value of 2 ms "> is out of range of all other findings. 
*** In vitro value = 1 ms." 
the absolute excitation threshold, expressed in 
terms of voltage or current strength, varies 
with the electrode area and the characteristics 
of the volume conductor surrounding the myo- 
cardium a better quantitative method applicable 
under all conditions should measure current 
density or voltage gradient per millimeter, at 
the myocardial level.^o Unfortunately, this 
method of measurement has not been applied 
widely, and the term "chronaxie" has therefore 
been used to characterize the strength-duration 
curve under all conditions of stimulation. This 
value indicates the shortest stimulus duration 
which will cause a response at a current 
strength twice that needed with a very long 
stimulus duration (rheobase). We have found 
that the chronaxie of the frog myocardium is 
about 2 msec, that of frog skeletal muscle 1.2 
msec and that of frog motor nerve 0.1 msec — 
all values considerably higher than those found 
with implanted ventricular pacemakers in hu- 
mans (Table I) . The low values in the frog are 
probably due to the relatively low temperature, 
since values in the dog or human taken from 
the cooled surface of the exposed heart or in 
isolated myocardial strips were found to be 
lower than in the case of implanted pacemakers 
(footnotes to Table I). 
The Chicken 
The chicken as an experimental animal is 
at present in use only for the study of em- 
bryonic muscle in tissue cultures. As in the 
case of isolated muscle strips, this method has 
the advantage of accessibility and of avoid- 
ing the unpredictable intervention of the home- 
ostatic mechanisms of the body. However, the 
tissue culture method allows the isolation of 
muscle cells which are free of nerves and are 
under natural conditions, without the effects of 
mechanical injury unavoidable during prepara- 
tion of a muscle strip. Since the cells arrange 
themselves in a single layer, they are not likely 
to suffer from hypoxia. This arrangement also 
allows exact control of the current distribution 
during stimulation and of the chemical and 
physical properties of the extracellular medium, 
and makes it possible to change these properties 
rapidly. Furthermore, mechanical contractile 
and electrical properties can be measured on 
the same cell while in strips or in situ hearts, 
electrical properties can be measured only in 
surface cells while the contraction curve is pre- 
dominantly the result of activity of the inner 
cell groups, which may be under different phys- 
iological conditions than the surface cells. 
Chicken embryos are said to have less myo- 
cardial potassium and more myocardial sodium 
than the adult birds, but their action and rest- 
ing potentials seem to be of a comparable mag- 
nitude.^^ One difference compared to mammals 
is that the muscle cells of birds are smaller in 
diameter and have no transverse tubules (Table 
I). The absence of these tubules in birds and 
amphibia is presumably related to the small fi- 
ber size, which could provide adequate diffusion 
