CHANGES IN A MUSCLE DURING CONTRACTION. 77 



Immediately upon removal from the body, the preparation possesses a 

 certain amount of irritability, not differing very materially from that which 

 the muscle and nerve possess while within and forming an integral part of 

 the body ; but after removal from the body the preparation loses irritability, 

 the rate of loss being dependent on a variety of circumstances ; and this goes 

 on until, since no stimulus which we can apply will give rise to a contrac- 

 tion, we say the irritability has wholly disappeared. 



We might take this disappearance of irritability as marking the death of 

 the preparation, but it is followed sooner or later by a curious change in the 

 muscle, which is called rigor mortis, and which we shall study presently ; and 

 it is convenient to regard this rigor mortis as marking the death of the muscle. 



The irritable muscle, then, when stimulated either directly, the stimulus 

 being applied to itself, or indirectly, the stimulus being applied to its nerve, 

 responds to the stimulus by a change of form which is essentially a shorten- 

 ing and thickening. By the shortening (and thickening) the muscle in con- 

 tracting is able to do work, to move the parts to which it is attached ; it thus 

 sets free energy. We have now to study more in detail how this energy is 

 set free, and the laws which regulate its expenditure. 



ON THE CHANGES WHICH TAKE PLACE IN A MUSCLE DURING A 



CONTRACTION. 



The Change in Form. 



52. An ordinary skeletal muscle consists of elementary muscle fibres, 

 bound together in variously arranged bundles by connective tissue which 

 carries bloodvessels, nerves, and lymphatics. 



The contraction of a muscle is the contraction of all or some of its ele- 

 mentary fibres, the connective tissue being passive ; hence while those fibres 

 of muscle which end directly in the tendon, in contracting pull directly on 

 the tendon, those which do not so end pull indirectly on the tendon by 

 means of the connective tissue between the bundles, which connective 

 tissue is continuous with the tendon. 



Each muscle is supplied by one or more branches of nerves composed of 

 medullated fibres, with a certain proportion of non-medullated fibres. These 

 branches running in the connective tissue divide into smaller branches and 

 t\vigs between the bundles and fibres. Some of the nerve fibres are dis- 

 tributed to the bloodvessels, and others end in a manner of which we 

 shall speak later on in treating of muscular sensations ; but by far the 

 greater part of the medullated fibres and in the muscular fibres, the 

 arrangement being such that every muscular fibre is supplied with at least 

 one medullated nerve fibre, which joins the muscular fibre somewhere about 

 the middle between its two ends or sometimes nearer one end, in a special 

 nerve ending, of which we shall presently have to speak, called an end- 

 plate. The nerve fibres thus destined to end in the muscular fibres divide 

 as they enter the muscle, so that what, as it enters the muscle, is a single 

 nerve 'fibre, may, by dividing, end as several nerve fibres in several muscu- 

 lar fibres. Sometimes two nerve fibres join one muscular fibre, but in this 

 case the end-plate of each nerve fibre is still at some distance from the end 

 of the muscular fibre. It follows that when a muscular fibre is stimulated 

 by means of a nerve fibre, the nervous impulse travelling down the nerve 

 fibre falls into the muscular fibre not at one end but at about its middle ; it is 

 the middle of the fibre which is affected first by the nervous impulse, and 

 the changes in the muscular substance started in the middle of the muscu- 

 lar fibre travel thence to the two ends of the fibre. In an ordinary skele- 



