THE HORSE IX MOTION. 29 



of the joints in the extremities. These cartilages by their elasticity 

 admit of slight flexion of the vertebra, and they also deaden the force 

 of the shock transmitted from the powerful impulses of the posterior 

 limbs. As has been already stated, the flexion is limited by the liga- 

 ments which bind them to each other. This restriction of motion is 

 necessary for the protection of the vital organs of the thorax and 

 abdomen, as well as the great nerve trunk transmitted through a 

 continuous canal above the bodies of the vertebra, and which is dis- 

 tributed thence to all parts of the body. 



While the three central divisions of the vertebra may be curved 

 slightly, they cannot be shortened, even temporarily, as may be readily 



:i ; and the apparent shortening that takes place when the animal's 

 limbs are gathered under him is an illusion. The elasticity of the 

 cartilages and ligaments is greatest in the young ; as age advances, 

 these tissues become stronger and less flexible, and resist the move- 

 ments of the joints; they are said to become "stiff." Hence the 

 importance of early training to give greater sweep and freedom of 

 motion. This physiological principle is made the basis of gymnastic 

 training by acrobats, being commenced at a very early age; and the 

 same is not lost sight of in the exercises of colts. 



In contemplating the passive parts of the animated machine ab- 

 stractly, we see the results of organic life ; they are without sensi- 

 bility or power of spontaneous motion ; we are familiar with the 

 mechanical principles involved in their action, and are impressed by 

 the perfect adaptation of means to ends ; we look upon them as we 

 look upon the piston, connecting-rod, and crank of a steam-engine : 

 but upon the muscles we look with far different thoughts ; their action 

 has no similitude in the inanimate world. 



The general appearance of muscle is too familiar to every one to 

 need description ; its special vital property is contractility. The mus- 

 cles are both voluntary and involuntary, but it is only the former that 

 are concerned in locomotion. 



If we remove a fragment of muscle from an animal recently killed 

 and examine it closely, we shall find it to be made up of longitudinal 

 fibres of a red color bound together by gray fibres of a different tissue. 



