200 



STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



form, and to be made up of a linear aggregation of distinct cells. We 

 observe the same alternation of light and dark spaces, as when the 

 fibrillae are united into fibres or into small bundles ; but it may be 

 distinctly seen, that each light space is divided by a transverse line ; 

 and that there is a pellucid border at the sides of the dark spaces, as 

 well as between their contiguous extremities (Fig. 63). This pellucid 

 border seems to be the cell-wall ; the dark space enclosed by it (which 

 is usually bright in the centre) being the cavity of the cell, which is 

 filled with a highly-refracting substance. When the fibril is in a state 

 of relaxation, as seen at a, the diameter of the cells is greatest in the 

 longitudinal direction : but when it is contracted, the fibril increases in 

 diameter as it diminishes in length ; so that the transverse diameter of 

 each cell becomes equal to the longitudinal diameter, as seen at b ; or 

 even exceeds it. Thus the act of Muscular contraction seems to con- 

 sist in a change of form in the cells of the ultimate fibril- 

 lae, consequent upon an attraction between the walls of 

 their two extremities ; and it is interesting to observe, how 

 very closely it thus corresponds with the contraction of 

 certain Vegetable tissues, of which the component cells 

 ( 345) appear to produce a movement, when they are 

 irritated by means of a similar change of form. The 

 essential difference, therefore, between the muscular tissue 

 of Animals, and the contractile tissues of Plants, consists 

 in the subjection of the former to nervous influence ( 353). 

 The diameter of -the ultimate fibrillae will of course be sub- 

 ject to variations, in accordance with their contracted or 

 relaxed condition; but seems to be otherwise tolerably 

 uniform in different animals, being for the most part about 

 l-10,000th of an inch. It has been observed, however, 

 as high as l-5000th of an inch, and as low as l-20,000th, 

 even when not put upon the stretch. The average dis- 

 tance of the striae, too, is nearly uniform in different ani- 

 mals ; though considerable variations present themselves 



different parts of the same 



cuiar fibre: a, a "*"e^c. JLHUB tuu maximum distance varies in different 

 animals from l-15,000thto l-20,000th of an inch; the mini- 



Structure of the 



ultimate fibriiise in every individual, and in 



of striated mus- muc^l^ Time tlin 

 cular fibre :-a, a n ' l6 ' lni 

 fibril in a 



tion; fc.afibriTin mum from l-7500th to l-4500th of an inch; while the 

 contrition?" al mean does not depart widely in any instance from 



i-10,000th. . 



337. The Muscular tissue of Organic life is very different from that 

 which has been now described. It exists under two forms; that of 

 fibres and that of cells. The fibres are distinguished from the prece- 

 ding by the absence of transverse markings, but appear to be tubular, 

 their contents having a granular existence, without any definite ar- 

 rangement of the particles into disks or fibrillse. Their size is usually 

 much less than that of the striated muscular fibre ; but owing to the 

 extreme variation in the degree of flattening which they undergo, it is 

 difficult to make even an average estimate of their dimensions. Those 

 of the alimentary canal of Man are stated by Dr. Baly to measure 

 from about the l-2500th to the l-5600th part of an inch in diameter. 



