

THE PHENOMENA OF MUSCLE AND NERVE. 55 



muscle, is technically called a contraction of the muscle ; and the muscular 

 tissue is spoken of as a contractile tissue. The heart is chiefly composed of 

 muscular tissue, differing in certain minor features from the muscular tissue 

 of the skeletal muscles, and the beat of the heart is essentially a contraction 

 of the muscular tissue composing it, a shortening of the peculiar muscular 

 fibres of which the heart is chiefly made up. The movements of the ali- 

 mentary canal and of many other organs are similarly the results of the 

 contraction of the muscular tissue entering into the composition of those 

 organs, of the shortening of certain muscular fibres built up into those 

 organs. In fact, almost all the movements of the body are the result of 

 the contraction of muscular fibres, of various nature and variously disposed. 



Some few movements, however, are carried out by structures which can- 

 not be called muscular. Thus, in the pulmonary passages and elsewhere, 

 movement is effected by means of cilia attached to epithelium cells ; and 

 elsewhere, as in the case of the migrating white corpuscles of the blood, 

 transference from place to place in the body is brought about by amoeboid 

 movements. But as we shall see the changes in the epithelium cell or white 

 corpuscle which are at the bottom of ciliary or amoeboid movements are, in 

 all probability, fundamentally the same as those which take place in a mus- 

 cular fibre when it contracts : they are of the nature of a contraction, and 

 hence we may speak of all these as different forms of contractile tissue. 



Of all these various forms of contractile tissue, the skeletal muscles, on 

 account of the more complete development of their functions, will be better 

 studied first ; the others, on account of their very simplicity, are in many 

 respects less satisfactorily understood. 



All the ordinary skeletal muscles are connected with nerves. We have 

 no reason for thinking that they are thrown into contraction, under normal 

 conditions, otherwise than by the agency of nerves. 



Muscles and nerves being thus so closely allied, and having besides so 

 many properties in common, it will conduce* to clearness and brevity if we 

 treat them together. 



THE PHENOMENA OF MUSCLE AND NERVE. 



Muscular and Nervous Irritability. 



40. The skeletal muscles of a frog, the brain and spinal cord of which 

 have been destroyed, do not exhibit any spontaneous movements or contrac- 

 tions, even though the nerves be otherwise quite intact. Left undisturbed, 

 the whole body may decompose without any contraction of any of the skele- 

 tal muscles having been witnessed. Neither the skeletal muscles nor the 

 nerves distributed to them possess any power of automatic action. 



If, however, a muscle be laid bare and be more or less violently disturbed 

 if, for instance, it be pinched, or touched with a hot wire, or brought into 

 contact with certain chemical substances, or subjected to the action of gal- 

 vanic currents it will move, that is, contract, whenever it is thus disturbed. 

 Though not exhibiting any spontaneous activity, the muscle is (and con- 

 tinues for some time after the general death of the animal to be) irritable. 

 Though it remains quite quiescent when left untouched, its powers are then 

 dormant only not absent. These require to be roused or " stimulated " by 

 some change or disturbance in order that they may manifest themselves. 

 The substances or agents which are thus able to evoke the activity of an 

 irritable muscle are spoken of as stimuli. 



But to produce a contraction in a muscle the stimulus need not be ap- 

 plied directly to the muscle ; it may be applied indirectly by means of the 



