52 PHYSIOLOGY OF THE HUMAN BODY. 
2-4. (PHYSICS, p. 11) ; the fulcrum is generally a joint ; the power is a muscle, 
the force being exerted by contraction, and the weight the farther end of 
the bone or limb. They are all levers of the third kind (Puystics, p. 
9), the power acting between the weight and the fulcrum, and therefore 
acting at a mechanical disadvantage—that ls, a 
power greater than the resistance has to be 
exerted in order to accomplish the movement. 
There is another disadvantage which the muscles. 
labour under in working, arising from their 
being attached to the bones in an_oblique 
direction ; but this is partly done away with by 
Fig. 48, means of the peculiar formation of the bones. 
—that is, by a thickening at the joints. Thus, 
in fig. 48, A, when a bone (0) is to be moved, the tendon (z) of the muscle 
(m) comes over the thickened part of the bone at the joint, and is attached 
immediately below it, so that the attachment of the muscle is more nearly 
perpendicular than it would otherwise have been. By this means the 
bones turn more freely on each other ; otherwise the ends of two bones 
would merely be pressed together, as in fig. 48, B. As regards the 
mechanical disadvantage from the nature of the lever, it is compensated 
for, we know, by the gain in velocity (PHysics, p. 10), as will be seen 
from the consideration of one case, the raising of the fore-arm. The lever 
to be raised is the fore-arm, bc, the weight being whatever is in the hand; 
the fulcrum is at the elbow-joint ; the power is in the muscle d, inserted 
at e. Now, it is clear that 
when the muscle is con- 
tracted a little, the hand will 
be raised a distance propor- 
tioned to the length of the 
arm, and, of course, must 
move proportionally quicker ; 
while, at the same time, the 
power exerted by the hand 
requires an exertion on the 
part of the muscle greater 
Fig. 49. exactly in the same pro- 
portion (PHysics, page 11). 
This is the general principle on which the muscles of the body act. 
Principal Muscles of the Body.—Space will not permit a detailed descrip- 
tion of the various muscles of the body: a few of the principal muscles 
only can be mentioned. The various emotions of pleasure, anger, wonder, 
scorn, &c., are expressed in the face by a smile, a frown, a raising of the 
eyebrows, a curl of the lip, &., which are caused by the action of the 


