MUSCULAR TISSUE. 



Fig. LXXII. 



firmer parts in the neighbourhood. Small tendons are also fixed by blending 

 with the fibrous sheaths investing contiguous muscular bundles. In the 

 gullet, Mr. Ellis has found that the longitudinal muscular fasciculi are inter- 

 sected wholly or partially, at intervals of from ^V to -J^ of an inch, by 

 small tendons into which they are inserted, after the fashion of the rectus 

 abdomiiiis, only on a miniature scale, and he thinks it probable that this 

 disposition may exist in other parts. 



The plain muscular tissue is met with iu the lower half of the gullet, the 

 stomach, and the whole intestinal canal ; that is, both in the muscular coat 

 of the alimentary canal, and also as a layer in the tissue of the mucous 

 membrane, and in the villi ; in the trachea and bronchial tubes, in the 

 bladder and ureters and the ducts of the larger glands generally, in the 

 uterus and its appendages, in the corpora caveruosa of both sexes, in the 

 prostate gland, and in the ciliary muscle and iris. The middle coat of the 

 arteries, the coats of many veins and the larger lym- 

 phatics contain plain muscular tissue. It has also been 

 detected in certain parts of the skin, in the dartos or 

 subcutaneous tissue of the scrotum, and in form of 

 minute muscles attached to the hair-follicles. 



Muscular tissue of the heart. The fibres of the heart 

 differ remarkably from those of involuntary muscular 

 organs in general, inasmuch as they present transverse 

 strise. The striae, however, are less strongly marked, 

 and less regular, and the fibres are smaller in diameter 

 than in the voluntary muscles. Many of the fibres are 

 attached to the tendinous structure connected with the 

 orifices and valves, and as has been already stated, they 

 are seen to divide and anastomose (fig. LXXII). The 

 tissue of the heart differs also from most other involun- 

 tary muscular structures by its deep colour, but it 

 agrees with them in the interlacement of its fasciculi. 



Development of muscle. The form-elements of the 

 plain or uii striped muscular tissue are derived from 

 embryonic nucleated cells, consisting of granular proto- 

 plasmic 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 coalescence of 

 several cells ; and this opinion is still maintained by some authorities. 

 Recent researches, however, for the most part, tend to establish the view 

 originally, I believe, promulgated by Reinak, viz., that the fibres are pro- 

 duced by the elongation of single cells, with differentiation of their contents 

 and multiplication of their nuclei ; and Dr. Wilson Fox, who has quite lately 

 investigated the process in the tadpole, the chick, and the mammalian 

 embryo, at very early stages, has arrived at the same conclusion.* Dr. Fox 

 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 with pointed ends ; the nucleus 



Fig. LXXII. MUS- 

 CULAR FIBRES 

 FROM THE HEART, 

 MAGNIFIED, SHOW- 

 ING THEIR CROSS 

 STRI.E, DIVISIONS, 

 AND JUNCTIONS 

 (from Kolliker). 



Phil. Trans. 1866, p. 101. 



