HAP. v.] THE CAT'S MUSCLES. 129 



Some muscles move a bone away from a given axis, and are 

 therefore termed abductors. Others tend to bring it towards such 

 an axis, and such are called adductors. The epithets "protractors," 

 "retractors" "elevators," and "depressors" (terms which require 

 no explanation), are also sometimes made use of. 



There cannot, however, be any really good classification of muscles 

 according to the functions they execute, because such functions may 

 vary in different animals with regard to the very same muscle. 



5. In considering the action of muscles, the SUPPORT OF THE 

 BODY may be first considered. The way in which this is affected 

 varies of course with the posture it may assume. In standing, the 

 basis of support afforded by the four paws is very wide, but the 

 posture cannot be maintained when the muscles are inactive, on 

 account of the flexibility of the joints. It is maintained by the 

 normal contraction (tonicity) of the muscles, which, being placed on 

 opposite sides of the body and of each supporting limb, antagonize 

 each other, and so prevent the joints from flexing, and the body 

 from consequently drooping, collapsing, and falling to the ground, 

 as it does immediately when any sudden cause (such as a violent 

 blow on the animal's head, or shot sent through its heart) suddenly 

 suspends their action. 



During waking life, changes of posture, which tend to cause the 

 centre of gravity to fall without the basis of support, are instinctively 

 followed by compensating motions which have the effect of retaining 

 it within such basis. Thus if the left fore-leg be extended outwards 

 to the left, the body instinctively and simultaneously leans, or the 

 tail is thrown, over to the right, and the extreme mobility of the 

 tail in all directions is a great agent in maintaining the equilibrium 

 of the body. 



In locomotion the limbs may be used either successively, as in 

 walking, or simultaneously as in springing and running. In leaping, 

 all the joints of the hind-limbs are bent, and these by their sudden 

 unbending give impetus to the body. 



In walking, each leg alternately swings forward as a pendulum, 

 the fore and hind-limb of each side being advanced successively and 

 alternately with one of those of the opposite side, as e.g., 1st, left 

 fore-leg ; 2nd, right hind-leg ; 3rd, right fore-leg ; 4th, left hind- 

 leg. Even in walking, however, the impetus is imparted by the 

 hinder limbs, the action of the fore-limbs being mainly that of 

 support. 



The part of the foot applied to the ground by the cat does not 

 answer to the sole of the human foot, but only to the toes ; the 

 heel being raised much above the ground. Similarly in the fore- 

 foot the wrist is raised and the digits alone support the body. On 

 this account the cat's mode of progression is spoken of as 

 digitigrade. 



As has been already said, the muscles generally act on the bones, 

 making use of the latter as LEVERS or FULCRA. 



The levers used in the cat's body are of all the three orders 



