MUSCLE 



203 



Even a cursory examination of mam- 

 malian muscles shows that those of the 

 trunk and limbs, skeletal muscles, are 

 different from those of such internal organs 

 as the bladder, uterus, and alimentary canal, 

 visceral muscles. 



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1. The visceral muscles appear to be 

 formed from cells similar to ordinary con- 

 nective tissue cells. These elongate and be- 

 come deiinitely longitudinally fibrillated. 

 They thus become spindle-shaped cells, 

 varying in length from about 50 to 200 

 micro - millimetres. A delicate coverinsr 

 membrane, the sarcolemyna, is said to be 

 present, but bridges of protoplasm may 

 extend from one fibre to another. In 

 mammals the nucleus is usually long, 

 almost rod-shaped, and is independent of 

 the fibrilla3 of the protoplasm. The nerves 

 which pass to these muscles are post- 

 ganglionic, and they form a plexus be- 

 \- / tween the fibres. In many situations, 



e.g. the wall of the intestine, well- 

 developed peripheral nerve ganglia are 

 present. 



m SSI 



Fig. 101.— (a) Some 

 of the sarcous 

 substance of a 

 fibre of skeletal 

 muscle teased 

 to show the 

 constituent 

 fibrils (sarco- 

 stj'les) with 

 transverse 

 markings ; (b) 

 dim band ; (c) 

 clear band with 

 Dobie's line. 



B. 



Fig. 102.— To illustrate 

 the possible structure 

 of a fibril as a series of 

 potential spheres in 

 the relaxed and in the 

 contracting condition. 



2. The skeletal muscles develop 

 from a special set of cells, early differ- 

 entiated as the muscle-plates in the 

 mesoblast down each side of the 

 vertebral column of the embryo. 

 Each cell elongates. The nucleus 

 divides across, but the cell, instead of 

 also dividing, lengthens, and continues 

 to elongate as the two daughter nuclei 

 again divide. A longitudinal fibrilla- 

 tion develops in the protoplasm, and 

 a series of transverse markings appears 



