Rev. S. Haughton on Animal Mechanics. 297 
If this estimate of the cross section of the muscles be as- 
sumed instead of my own, the coefficient found by me should be 
increased in the proportion of 3190 to 2474; or 
Coefficient of muscles of forearm . . 94°7X 3190 _ 122 lbs. 
24:74 
The mean of the coefficients found from my own measurement 
of the muscles of the arm, and that of Professor Donders, is 
108-4 lbs., which agrees nearly with that obtained from the 
muscles of the leg, viz. 110°41bs., and the mean of all the ob- 
servations on arm and leg would be 109°4|bs., a result which I 
consider to be not far from the truth. 
The cross sections of the muscles were found by cutting them 
across with a sharp scalpel, and marking out their section on 
cardboard, and afterwards weighing the marked portions, the 
weights of which were then compared with the weight of a 
known number of square inches of the same cardboard, and so 
the cross sections in square inches calculated. 
I give here, for the purpose of illustration, the actual sections 
of the muscles of the leg. (Figs. 1-6.) 
The perpendiculars let fall upon the directions of the muscles 
were measured by stretching strings from the origin to the in- 
sertion of the muscles, and measuring, by means of a compass, 
the perpendiculars let fall upon these strings from the axis of the 
joint. 
The weights of the muscles themselves were as follows :— 
OZ. OZ. 
1. Biceps humeri . . 4°22 | 5. Semimembranosus . 7:25 
2. Bracmaus. . . . 504|6>Gracths . .. . 298 
3. Biceps femoris . .10°74| 7. Sartorius . . . . 566 
4. Semitendinosus . . 5°17 
ii. The principle of economy of force or of material in nature 
would lead necessarily to the principle that each tendon convey- 
ing the effect of a force to a distant point should have the exact 
strength required, and neither more nor less; for, according to 
the doctrine of final causes, it was originally contrived by a per- 
fect architect, and according to Lamarckian views it must have 
perfectly accommodated itself to the uses to which it is applied. 
According, therefore, to either view, if the tendon be too strong, 
it will become atrophied down to the proper limit; and if too weak, 
it must either break or be nourished up to the requisite degree 
of strength. It seemed to me desirable to prove this fundamental 
proposition in animal mechanics by direct observation; and I 
selected for this purpose the tendons in the leg of several of the 
large running birds (Struthionide),—and always with the same 
result, viz. that the cross sections of any two muscles tending to 
produce a similar effect are directly proportional to the cross 
sections of their tendons. 
_ Tshall select as an example the case of the flexor hallucis longus 
and flexor digitorwm communis perforans of the Rhea, whose 
Ann. & Mag. N. Hist. Ser. 3. Vol. xx. 
