242 



ANIMAL MECHANICS, 



the over-extension of the muscles of the back of the thigh thereby 

 induced, and the weight of the head which plumbs in front of the 

 atlanto-occipital joint is partly supported by the mechanically stretched 

 muscles at the back of the neck. 



An ordinary stretched muscle is, from a histological point of view, a 

 complex of muscular fibres and connective tissue, consisting for the most 

 part of the inextensible white variety. It is on this account that they 

 are able to resist over-extension, and it will be found that those muscles 

 which are frequently called upon to do this, which have in fact to 

 perform the part both of ligaments and muscles, have powerful 

 tendinous prolongations passing through them. In some vertebrates 

 this is very especially the case ; for instance, in the horse, where the 

 flexor metatarsi muscle is really a combination of a muscle and a check 

 ligament. In this muscle there is a long tendon, which arises from the 



anterior and inferior 

 portion of the femur, 

 and passes over the 

 stifle (knee) down the 

 front of the tibia and 

 over the hock (ankle), 

 and is inserted into the 



\ 7\ metatarsal bone. As 



\J N, ^ this tendon passes over 



two joints, the knee and 

 the ankle, when one of 



c' 



these is 

 one is 

 purely 

 manner. 



bent the other 



bent too, in 



a mechanical 



This round 



FIG. 130. Two curves are here represented ; the upper 

 one (abcde) is the extension of a tetanised muscle by 

 10, 20, 30, etc., grms. The lower curve (a'b'c'd'e') 

 represents the extension by the same weights of a 

 muscle at rest. The two curves are seen to cross 

 towards their lowest points, when the muscles are 

 extended by weights a little over 50 grms. 



inextensible cord re- 

 ceives the attachment 

 of a strong band of 

 muscular fibres spring- 

 ing from the tibia 

 itself, and is directly 

 pulled upon by the 



force of its muscular contraction. 



When a muscle with parallel fibres contracts and lifts a load, each 

 muscular fibre bears its fraction of the load, so that if the load be w 



iu 

 and the number of fibres n, each fibre raises a load of -. It follows that 



71 



the load which can be lifted will vary with the number of fibres, and 

 therefore with the sectional area of the gross muscle. When the fibres 

 are not parallel but obliquely set, as in the gastrocnemius, we have a 

 greatly extended transverse area of muscular fibres, which act therefore 

 very powerfully, though, on account of their short length, they can 

 exercise their pull through but a comparatively short distance. 



If a weight be attached to a muscle, and if that muscle be stimulated 

 to contract, it will exert a pull or force upon the weight, the mass of 

 which will then be acted upon in opposite directions by two forces, that 

 of gravity and the muscular force. The absolute muscular force may be 

 measured, after Weber, by so adjusting the mass of the load, that the 

 muscle on passing into activity neither lifts the load nor is itself 



