416 
Arnott, though it has been erroneously ascribed 
by Professor Miiller to Weber. Subsequent 
experiments made by Dr. Todd, Mr. Wormald, 
and others, have fully established the mecha- 
nical influence of the air in keeping the mecha- 
nism of the joints together. The amount of 
atmospheric pressure on any joint depends upon 
the area or surface presented to its influence 
and the height of the barometer. The forces 
acting in opposition to the weight of the limb 
are the pressure of the air, and a force em- 
ployed by the ligaments and muscles equal to 
the excess of the weight of the limb, if any, 
above that of the pressure of the atmosphere. 
According to Weber the atmospheric pressure 
on the hip-joint of a man is about 26 pounds, 
The pressure on the knee-joint is estimated by 
Dr. Arnott at 60 pounds. This estimate agrees 
with our measurement of the area of the surface 
of the knee-joint in an adult female, but is too 
small for an adult male which is about 90 
pounds. In the Elephant and Megatherium, 
the pressure of the air upon the joints is 
greater in proportion to the increased bulk and 
weight of their limbs. In the latter the area 
of the plane bounded by the edge of the coty- 
loid cavity, is an ellipse whose diameters are 
seven and eight inches, and therefore present 
a plane surface exceeding 43.9824 square inches, 
which, being multiplied by fifteen, with the 
barometer at 30inches, will be =43.9824 x 15, 
or about six hundred and sixty pounds pres- 
sure upon the cotyloid joint of this greatest 
of terrestrial Mammalia. 
Muscles—The amount and effects of mus- 
cular contraction, the absolute and relative 
power of muscles in reference to their length, 
mass, and obliquity of direction, and the ex- 
penditure of their power to support given 
weights, will now be either simply noticed or 
briefly estimated. 
In the application of muscles to the purposes 
of locomotion we find them so arranged as to 
produce great velocity, and at the same time to 
admit a great extent of motion, still preserving 
the beauty of proportion. These objects are 
obtained, ist, from the oblique directions of 
their fibres towards the tendons; 2d, from the 
obliquity of the direction of the tendon to the 
bones on which they act ; 3d, from the proxi- 
mity of their points of insertion to the articu- 
lations of the bones, or axes of motion in the 
joints. 
The muscles are capable of «contracting (ac- 
cording to the researches of MM. Prevost and 
Dumas) about #, or nearly one-fourth their 
whole length, which, owing to the circumstances 
just mentioned, is sufficient to produce all the 
positions and motions observed in animals. 
It has been already determined by experi- 
ment that the volumes of muscles do not alter 
by contraction, their thickness only increasing 
as they decrease in length, and vice versa. 
The comparative power of muscles in the 
same animal, according to Borelli, may be 
thus estimated :— 
When two muscles are composed of an equal 
number of fibres, or are of equal thickness but 
of unequal lengths, they will suspend equal 
MOTION. 
weights, but their motor powers and the height 
to which they are capable of raising the weights 
will be as the lengths of the muscles. at 
2d. When the lengths of the muscles are 
equal and their thicknesses unequal, their rela~ 
tive powers will depend upon their thicknesses, — 
but they will raise weights to equal heights. 
3d. When the lengths and thicknesses a 
unequal, the weights they will raise will depend 
on their thicknesses, and the heights to whic 
they will raise them will be as their lengths. 
When the fleshy fibres of a muscle lie pa. 
rallel to the tendon, the s which 
they will draw it equals the contraction of the 
fleshy fibres; but when ae’ = are inserted ob- 
liquely into the tendon, space ugh 
which they will draw it will vary with hee vr 
clination. _ 
Thus, let two equal fleshy fibres, AC, B mi j 
Png. 
with 
and produce D C to meet it in P, 
Sp apiece to AB. Now if the poin 
be considered as fixed, and the angle A 
be such that radius : its sine : : AC ; 
length of A C when contracted, then the joint 
action of the fibres will draw the point C to P. 
Fig. 218. 
D 
For with A B, as centres describe the circular 
arcs P E, P F, touching each other at P, then 
it is evident that the point C will, after the _ 
contraction of C A, be somewhere in the are 
E P, because the radius of E P is the . 
of A C when contracted; for a similar a 
C will be somewhere in F P; therefore it 
will be at P, their point of contact. The 
same result becomes apparent from the consi- — 
deration that the forces in the direction C A 
C B are equivalent to forces in the direction 
C P, P Aand C P, P B respectively, of which 
the forces in the direction C P are not counter= 
acted, but gradually diminish and become 2 
when the fibres are at right angles to r 
tendon, that is, when C coincides with P. 
It is here assumed that there is no ob- 
stacle to the free motion of the tendon in 
the line C P, ey 
If the obliquity of the fibres be Jess than 
AC P, the arcs will intersect in some point 
contrary, the obliquity be greater than dl e - 
angle AC P, the arcs will not meet, but C 
