120- MUSCULAR TISSUE. 



situate near the centre ; occasionally two niaclei are seen. The cell 

 substance is faintly striated longitudinally as well as transversely : it 

 presents no indication of an investing membrane or sarcolemma. 



The mufcular fibres of the heart freely divide and anastomose (fig, 

 79), the junctions with neighbouring fibres being effected by the 

 medium of the cell-offsets above noticed. 



DEVELOPMENT AND GROWTH OF MUSCLE. 



Development. — The elements of the plain or toistriped muscuJar 

 f issue are derived from embryonic nucleated cells, consisting of granu- 

 lar protoplasmic substance, as usual. These become lengthened out, 

 pointed at the ends, and flattened, with elongation of the nucleus, 

 whilst their substance becomes more uniform in aspect, and acquires 

 its permanent condition and characteristic properties. 



The striated muscular tissue is also developed in the embryo from 

 cells. Schwann considered each fibre to be formed by the linear coa- 

 lescence of several cells ; recent researches, however, for the most part, 

 tend to establish the view, originally promulgated by Remak, 

 that the fibres are produced by the elongation of single cells, with 

 differentiation of their contents and multiplication of their nuclei. 

 Wilson Fox, who has lately investigated the process in the tadpole, the 

 chick, and the mammalian embryo, at very early stages, finds that the 

 first elements of the muscular fibres are rounded or oval cells, with a 

 clear nucleus and granular contents, agreeing in all respects with the 

 cells of which the parts of the embryo body originally consist. To 

 form a muscular fibre, a cell elongates, often acquiring pointed ends ; 

 the nucleus generally divides into two, and by further division these are 

 multiplied ; a fine membrane, at first absent or invisible, is soon 

 discovered, bounding the cell and enclosing its contents. In the 

 meantime the substance becomes striated longitudinally at one part, 

 and more transparent, the granules disappearing. The striation, 

 which is the first indication of the proper muscular substance, extends 

 throughout the length of the elongated cell, but at first affects only 

 a small part of its breadth, and the remaining space is occupied by 

 unchanged granular matter and the nucleus or nuclei which lie on one 

 side. In due time, however, this conversion into the proper muscular 

 substance, further shown l\y the appearance of cross stria?, proceeds 

 through the whole thickness of the cell, or fibre as it may now be 

 called ; the enclosing cell-membrane becomes the sarcolemma, and 

 the nuclei, with a small residue of the granular protoplasm still 

 adhering to them, remain as the muscle corpuscles.* 



Growth. — The muscular fibres of the growing foetus, after having 

 acquired their characteristic form and structure, continue to increase 

 in size till the time of birth, and thenceforward up to adult age. 

 In a full-grown foetus most of them measure twice, and some of them 

 three or four times their size at the middle of foetal life ; and in 

 the adult they are about five times as large as at birth. This increase 

 in bulk of the individual fibres would, of course, so far account for the 

 concomitant enlargement of the entire muscles. But there would seem 

 to be also a multiplication of the fibres ; and Budge believes he has 



* Phil. Tran?. 186C. 



