LOCOMOTION. 655 
LOCOMOTION. 
The entire locomotor system of tissues is derived from the 
embryonic mesoblast. These include the muscles, bones, carti- 
lage, and connective and fibrous tissues; and the tissues that 
make up the vascular system or the motor apparatus for the 
circulation of the blood. Locomotion in the mammal is effected 
by the movement of certain bony levers, while the equilibrium 
of the body is maintained. The whole series of levers is bound 
together by muscles, tendons, ligaments, etc., and play over > 
one another at certain points where they are invested with car- f 
tilage, and kept moist by a se- , 
cretion from the cells covering 
the synovial membranes that 
form the inner linings of joints. 
Cartilage, a very low form 
of tissue destitute of blood-ves- 
sels, and hence badly repaired 
when lost by injury or disease, 
forms a series of smooth sur- 
‘faces admirably adapted for 
joints, and especially fitted to 
act as a series of elastic buffers, 
and thus prevent shocks. Bone, 
‘though brittle in the dried state, 
possesses, when alive, a favora- 
ble degree of elasticity, while 
ufficiently rigid. Provision is 
made by its vascular periosteum 
aud central marrow (in the case 
of the long bones), as well as by 
the blood-supply derived from 
the nutrient artery and its rami- 
fications throughout the osseous tissue, for abundant nourish- 
ment, growth, and repair after injury. 
We find in the body of mammals, including man, examples 
of all three kinds of levers. It sometimes happens that there 
is an apparent sacrifice of energy, the best leverage not being 
exemplified; but on closer examination it will be seen that the 
weight must either be moved with nice precision or through 
large distances, and these objects can not be accomplished al- 
ways by the arrangements that would simply furnish the most 
Fia. 488. 
