400 THE TISSUES. 



homogeneous or a finely granular aspect, and, rarely, exhibit a faint 

 trace of transverse striation. In the fourth month, they are mostly 

 from ^es to 24 -V^ f an mcn i n diameter. The larger ones, though 

 still flattened, are now of uniform width, thicker than before, 

 transversely striated, and with fibrils capable of being isolated. 

 (Fig. 253, 3). The musculine does not, however, yet entirely fill the 

 myolemma, but forms a tube in contact with its inner surface, con- 

 taining some of the original contents of the myolemma in its cen- 

 tre. Thus the musculine is the part of the fibre which is last de- 

 veloped. The myolemma may occasionally be raised like a very 

 delicate membrane, by the imbibition of water. The nuclei still lie 

 close upon the myolemma, as at first, and are rapidly multiplying ; 

 being much more numerous than at first, and often found in groups 

 of three or four, or even six, which are sometimes arranged serially. 

 They are all vesicular, with very distinct, simple, or double nucleoli, 

 and frequently with two secondary cells in their interior, showing 

 the endogenous development of the nuclei from the original ones. 

 At birth, the fibres are 2742 to jsV? f an i ncn i n diameter; are 

 solid, rounded, polygonal, and longitudinally and transversely stri- 

 ated, as in the adult; and the nuclei have disappeared. (Fig. 253, 5.) 



Thus the myolemma represents the sum of the membranes of the 

 original coalesced cells, and the fibrillaB are the altered contents of 

 the original tubes (myolemmata). A fibre, therefore (and not a 

 fibrilla, as Leidy and Reichert maintain), is histologically analogous 

 to a contractile fibre-cell (p. 390); and the latter may be regarded 

 as a lower development of the former. 



The growth of the striated muscular fibre, must be referred prin- 

 cipally at least, to increase in the number of the fibrillae, and of 

 course of the size of the myolemma containing them. In other 

 words, each fibre grows larger, while there is no proof that new 

 fibres are formed even after the middle period of intra- uterine life. 

 Thus the fibres are about five times as thick in an embryo at four 

 or five months as at two months; and three or four times as thick 

 in the new-born infant as at the period first mentioned. In the 

 adult, they are perhaps five times as thick as at birth. Bonders 

 thinks the number of the fibrillce, is the same in the young and the 

 adult animal, and that they only increase in size ; they being J to 

 f smaller in the calf than in the ox. Kolliker, however, relying 

 on Harting's assertion that they are but little thicker in the adult 

 than in the foetus, believes their number increases in each fibre. 



