TISSUES OF THE BODY. 281 



physiology, the contraction of muscular tissue is of two kinds, voluntary 

 and involuntary. 



Viewed from a histological point of view, muscles may be divided into 

 those which are made up of long transversely striated fibres as elementary 

 structures (fig. 271), and those built up of smooth or unstriped fusiform 

 elongated cells (fig. 272). Dependent on these differences we speak of 

 striped and smooth muscle. 



This anatomical difference, however, seems at first sight much greater 

 than it is in reality. 



In the first place, we encounter many intermediate forms between these 

 two species of muscular tissue in the animal world ; and, secondly, the 

 history of development has recently shown that both elements have an 

 origin extremely similar, namely, each form a single cell ( 59). The 

 element of the unstriped tissue preserves this character throughout life, 

 the striped fibre forsakes its original nature in the greater complication 

 of its development. 



In conclusion, we need only remark that the voluntary muscles of our 

 body consist of striated fibres (the heart, however, also, among those organs 

 which are involuntary), whilst those muscles withdrawn from the influence 

 of the will are composed of smooth fibres. The expressions, therefore, of 

 "smooth" and "involuntary," or "striped" and "voluntary," do not 

 correspond exactly in the human body. The specific gravity of the first 

 of these was settled by Krause and Fischer to be 1'058, that of the latter 

 1-041. 



163. 



The elements of unstriped muscular tissue (fig. 273) were formerly held 

 to be long, pale, band-like fibres (i) t displaying at intervals several like- 

 wise elongated nuclei. It remained for Koelliker's quickness of perception 

 to recognise in these fibres a series of elongated cells arranged linearly one 

 after the other, and in the year 1847 to introduce to the notice of his- 

 tologists the " contractile fibre-cell" (c-7*), a great step towards a proper 

 comprehension of the structure of this tissue, so difficult of investigation. 



We usually meet with the smooth muscle-cell in the form of a long 

 (d-f) band, which may at times possess extremely great length (g), and 

 which generally runs off to a point at both ends. It is often short however 

 (c). Its medium length is about 0451-0'0902 mm., short cells measuring 

 often 0'0282 mm., and very long specimens 0'2256 mm. and upwards. 

 Its breadth lies between 0'0074 and 0'0151 mm. 



Further, it appears pale and homogeneous, either completely colourless 

 or tinged slightly yellow, and without recognisable difference between 

 the envelope and contents. Not unfrequently we may remark a row of 

 granules, the residue of the earlier protoplasm, extending from each pole 

 of the nucleus into the body of the cell (fig. 272, er) ; small dust-like 

 molecules of the same may also cloud the otherwise clear substance of the 

 cell. Finally, as a sign of retrograde metamorphosis, we find fat granules 

 in varying quantity and size (fig. 273, h). 



The contractile fibre-cell may present a very characteristic appearance, 

 principally due to its nucleus, which appears under the action of strong 

 acid as a tolerably pale, long, cylindrical rod, more or less rounded at both 

 ends. Again, this nucleus is met with quite homogeneous, without any 

 difference of contents and envelope, and apparently without any nucleus. 

 Its medium length is 0-0226 mm., and breadth 0'0023-0'0029 mm. Its 



