300 



DEVELOPMENT OF VOLUNTARY MUSCULAR TISSUE. 



In the deeper coloured muscular fibres of those animals which, like the rabbit, possess 

 two kinds of voluntary muscles, the transverse loops of the capillary network are dilated far 

 beyond the size of the ordinary capillaries. 



The number of capillaries in a given space of a muscle, 

 or their degree of closeness is partly regulated by the size 

 of the fibres ; and accordingly in the muscles of different 

 animals it is found that, when the fibres are small, the vessels 

 are numerous and form a close network, and vice versa : in 

 other words, the smaller the fibres, the greater is the quantity 

 of blood supplied to the same bulk of muscle. In conformity 

 with this, we see that in birds and mammalia, in which 

 the process of nutrition is active, and where the rapid 

 change requires a copious supply of material, the muscular 

 fibres are smaller and the vessels more numerous than 

 in cold-blooded animals, in which the opposite conditions 

 prevail. 



Lymphatics. So far as is known there are no 

 lymphatic vessels in the voluntary muscles, although 

 there is an abundant supply in their connective tissue 

 sheaths and tendons, and the lymphatic vessels here 

 would seem, as pointed out by Ludwig and Schweigger- 

 Seidel, to serve the purpose of collecting and convey- 

 ing away the lymph from the muscular substance. 



Nerves. The nerves of a voluntary muscle are 

 of considerable size. Their branches pass between 

 the fasciculi, and repeatedly unite with each other 

 in form of a plexus, which is for the most part con- 

 fined to a small part of the length of the muscle, or 

 muscular division in which it lies. From one or 

 more of such primary plexuses, nervous twigs proceed, 

 and form finer plexuses composed of slender bundles, 



each containing not more than two or three dark-bordered nerve-fibres, whence 

 single fibres pass off between the muscular fibres and divide into branches which are 

 finally distributed to the tissue. The mode of final distribution will be described 

 with the general anatomy of the nerves. 



DEVELOPMENT OF VOLUNTARY MUSCULAR TISSUE. 



Most of the voluntary muscles of the body are developed from a series of portions 

 of mesoblast which are early set aside for this purpose in the embryo and are termed 

 the muscle-plates (see Embryology, p. 159). When the muscular fibres are about to 

 be formed the cells become elongated, and their nuclei multiplied so that each cell is 

 converted into a long multi-nucleated protoplasmic fibre. At first the substance of 

 the fibre is not striated but is merely granular in appearance, but presently it 

 becomes longitudinally striated along one side (fig. 345, A), and about the same time 

 a delicate membrane, the sarcolemma, may be discovered bounding the fibre. The 

 longitudinal striation, which is the first indication of the proper muscular substance, 

 extends along the whole length of the fibre, but at first as just intimated affects only 

 a small part of its breadth, the rest being formed by a highly glycogenic protoplasm 

 containing the nuclei. In due time, however, this conversion into the proper 

 muscular substance, further shown by the appearance of cross striae (fig. 345, B 

 and c), extends round the greater part of the circumference of the fibre, and finally 

 gradually involves its whole thickness, except along the axis, which for some time 

 remains occupied by the nuclei embedded in undifferentiated protoplasm. Eventually, 

 however, the nuclei take up their permanent position. 



Schwann considered each fibre to be formed by the linear coalescence of several 

 cells ; but the researches of Kolliker, Wilson Fox, and others, tend to establish the 



Fig. 344. CAPILLARY VESSELS 



OF MUSCLE, MODERATELY MAG- 

 NIFIED. (E. A. S.) 



