208 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



mitted in the direction of its length, and being followed by relaxation. 

 Again, in the Muscular structure of the Bladder and Uterus, powerful 

 contractions are excited by irritation, and these produce a great degree 

 of shortening : but they do not alternate in the healthy state with any 

 rapid and decided elongation ; whilst, on the other hand, an irritation 

 applied to one spot causes more extensive contractions, than are seen to 

 occur as its immediate consequence in the preceding cases. In the 

 Heart, the muscular structure of a large part of the organ is thrown 

 into rapid and energetic contraction, by a stimulus applied at any one 

 point ; and this contraction is speedily followed by relaxation. And 

 in the fibrous tissue of the middle coat of the Arteries, the contraction 

 takes place rather after -the manner of that of the bladder arid uterus, 

 and a prolonged application of the stimulus is often necessary to produce 

 the effect ; but when the contraction commences, it produces a consi- 

 derable degree of shortening, which takes place in other fasciculi than 

 those directly irritated, and does not speedily give way to relaxation. 



353. On the other hand, when the stimuli which excite muscular 

 contraction are applied to the Nerve, which supplies a voluntary muscle 

 composed of striated fibre, they produce a simultaneous contraction in 

 the whole muscle; the effect of the stimulus being at once exerted 

 upon every part of it. In the ordinary action of such muscles, the 

 nervous system is always the channel through which they are called 

 into play, whether to carry into effect the determinations of the mind 

 ( 391), or to perform some office necessary to the continuance of life, 

 such as the movements concerned in Respiration ( 394). The nerves 

 of the striated fibre are all derived at once from the brain or spinal 

 cord. The ordinary actions of the non-striated fibre, on the contrary, 

 are executed in respondence to stimuli applied directly to themselves. 

 It is so difficult to excite contractions in it through the medium of its 

 nerves, that many Physiologists have denied the possibility of doing so ; 

 and the nerves lose their power of conveying the influence of stimuli 

 very soon after death, although the contractility of the muscles may 

 remain for a considerable time. The nerves of the non-striated fibre 

 are chiefly those belonging to the Sympathetic system ; but, as will be 

 shown hereafter (CHAP, xn.), those which excite motion are probably 

 derived in reality from the Cerebro-spinal system, through the commu- 

 nicating branches which unite the two. 



354. When a Muscle is thrown into contraction, its bulk does not 

 appear to be at all affected. Its extremities approach, so that it is 

 shortened in the direction of its fibres ; but its diameter enlarges in 

 the same proportion. It was formerly supposed that the ultimate 

 fibres, in the act of contraction, threw themselves into zigzag folds ; 

 but this is now well-ascertained not to be the case. The fibre, like 

 the entire muscle, preserves its straight direction in shortening, and 

 increases in diameter. The fibrillse themselves, as already mentioned 

 ( 336), exhibit an evident change, in regard to the distances of their 

 successive light and dark portions ; and the fibre, which is made up of 

 these, exhibits, in its contracted state, a very close approximation of 

 the transverse striae ; to such an extent that they become two, three, 

 or even four times as numerous in a given length, as they are in a 



