VERTEBRATED ANIMALS. 



283 



macerated in a weak solution of Chromic acid. The shape of the fibres 

 can only be properly seen in cross sections; and these are best made by 

 the Freezing Microtome ( 191). Striated fibres, separable x with great 

 facility into their component fibrillae, are readily obtainable from the 

 limbs of Crustacea and of Insects; and their presence is also readily dis- 

 tinguishable in the bodies of Worms, even of very low organization; so 

 that it may be regarded as characteristic of the Articulated series gener- 

 .ally. On the other hand, the Molluscous classes are for the most part 

 distinguished by the non-stratum of their fibre; there are, however, two 

 remarkable exceptions, strongly striated fibre having been found in the 

 Terebratula and other BracJiiopods (where, however, it is limited to the 

 anterior adductor muscles of the shell), and also in many Polyzoa. Its 

 presence seems related to energy and rapidity of movement; the non- 

 striated presenting itself where the movements are slower and feebler in 

 their character. 



FIG. 474. 



Diagram of Striated Fibrilla. 



Structure of non-stri- 

 ated Muscular Fibre: A, 

 portion of tissue showing 

 fusiform cells a, a, with 

 elongated nuclei 6, 6; B, a 

 single cell isolated and 

 more highly magnified; c, 

 a similar cell treated with 

 acetic acid. 



680. The ' smooth ' or non-striated form of Muscular fibre, which is 

 especially found in the walls of the stomach, -intestines, bladder, and 

 other similar parts, is composed of flattened bands whose diameter is 

 usually between l-2000th and l-3000th of an inch; and these bands are 

 collected into fasciculi, which do not lie parallel with each other, but 

 cross and interlace. By macerating a portion of such muscular sub- 

 stance, however, in dilute nitric acid (about one part of ordinary acid to 

 three parts of water) for two or three days, it is found that the bands 

 just mentioned may be easily separated into elongated fusiform cells, not 

 unlike 'woody fibre' in shape (Fig. 474, a, ); each distinguished, for 

 the most part, by the presence of a long staff-shaped nucleus, b, brought 

 into view by the action of acetic acid, c. These cells, in which the distinc- 

 tion between cell-wall and cell-contents can by no means be clearly seen, 

 are composed of a soft yellow substance often containing small pale 

 granules, and sometimes yellow globules of fatty matter. In the coats of 

 the Blood-vessels are found cells having the same general characters, but 

 shorter and wider in form; and although some of these approach very 



