CHAP, ii.] THE CONTRACTILE TISSUES. 159 



nucleus, gradually tapering away, and so forming a slender granular 

 core in the median portion of the fibre. 



The rest of the fibre, forming its chief part, is composed of a 

 transparent but somewhat refractive substance, which is either 

 homogeneous or exhibits a delicate longitudinal fibrillation; this is 

 the muscle substance of the fibre and corresponds to the muscle 

 substance of the striated fibre, but is not striated. Sometimes 

 the whole fibre is thrown into a series of transverse wrinkles, which 

 give it a striated appearance, but this is a very different striation 

 from that produced by an alternation of dim and bright bands. 

 No such alternation of bands is to be seen in the plain muscular 

 fibre ; the whole of the substance of the fibre around the nucleus 

 and core is homogeneous, or at least exhibits no differentation be- 

 yond that into fibrillse and interfibrillar substance, and even this 

 distinction is doubtful. 



The fibre has a sharp clear outline but is not limited by any 

 distinct sheath corresponding to the sarcolemma, at least according 

 to most observers. 



It is obvious that the plain muscular fibre is a nucleated cell, 

 the cell-substance of which has become differentiated into con- 

 tractile substance, the cell otherwise being but slightly changed ; 

 whereas the much larger striated fibre is either a number of cells 

 fused together or a cell which has undergone multiplication in so 

 far that its nucleus has given rise to several nuclei, but in which no 

 division of cell-substance has taken place. 



A number of such fusiform nucleated cells or fibres or fibre 

 cells are united together, not by connective tissue but by a peculiar 

 proteid cement substance into a flat band or bundle, the tapering 

 end of one fibre dovetailing in between the bodies of other fibres. 

 So long as this cement substance is intact it is very difficult to 

 isolate an individual fibre, but various reagents will dissolve or 

 lessen this cement, and then the fibres separate. Sometimes the 

 surface of the cell is not smooth, but thrown lengthwise into 

 ridges, the ridges of one cell abutting on those of its neighbours ; 

 in such cases, the amount of cement substance seems scanty. 



Small flat bands thus formed of fibres cemented together are 

 variously arranged by means of connective tissue, sometimes into a 

 plexus, sometimes into thicker larger bands, which in turn may be 

 bound up as we have said into sheets of varying thickness. 



In the plexus of course the bands run in various directions, 

 but in the sheets or membranes they follow for the most part the 

 same direction, and a thin transverse section of a somewhat thick 

 sheet presents a number of smaller or larger areas, corresponding to 

 the smaller or larger bands which are cut across. The limits 

 of each area are more or less clearly defined by the connective 

 tissue in which blood vessels may be seen, the area itself being 

 composed of a number of oval outlines, the sections of the flattened 

 individual fibres; in hardened specimens the outlines may from 



