CHAPTER LXI 

 THE STRUCTURE AND PHYSICAL PROPERTIES OF MUSCLE 



The Structure of Muscle. In the vertebrate animals, the muscle* 

 develop from cells which line the primitive ccelom or body cavity. 

 These cells become iiivaginated as buds from the coelomic surface, 

 to form a row of myomeres along each side of the animal. These 

 flatten, so as to form two plates, the inner of which gives rise to con 

 nective tissue, the outer to the striated muscle of the body. The cells 

 which are about to become muscle (the sarcoblasts) undergo a great 

 lengthening, and show signs of nuclear activity. The division of the 

 nucleus is amitotic there is no division of the cell body and many 

 nuclei are formed in one cell. 



At this stage fibrils, or, as the} 7 are sometimes termed, sarcostyles, 

 gradually appear in the sarcoplasm of the sarcoblast, faint at first, 

 but gradually becoming more distinct. They first appear on the inner 

 side of the cell, gradually pushing the nuclei to the outer side. The 

 formation of fibrils goes on until each cell appears a mass of fibrils, 

 with but little interfibrillar sarcoplasm. Each fibril is composed 

 of two kinds of substance, differentiated by staining and refractive 

 power. One substance the isotropic does not stain readily and 

 is singly refractile: the other the anisotropic substance stains 

 readily and is doubly refractile. Each substance is deposited alter- 

 nately with the other at regular intervals in the fibril. Thus, after 

 suitable treatment the fibril may be broken up into sarcomeres, or 

 sarcous elements. The lines of cleavage take place in the isotropic 

 substance; each sarcomere consists of a portion of anisotropic sub- 

 stance, with half a portion of isotropic substance on either side of 

 it. As all the sarcous elements of the neighbouring fibrils are in 

 perfect alignment isotropic with isotropic, anisotropic with aniso- 

 tropic the general effect is to give the fibres that cross-striped 

 appearance from which cross-striated muscle gets its name. 



The difference in the degree of differentiation of muscle is well 

 seen in the human body. The musculature of the trunk and limbs 

 (the skeletal muscles) differs from that of such internal organs as the 

 bladder, intestines, uterus (smooth or non-striated muscle), and also 

 from that of the heart (cardiac muscle). 



Voluntary or striated muscle is composed of a number of separate 

 fibres joined together to form the muscle. The character and number 

 of these fibres varies considerably with different muscles. In some 

 thev are pale and delicate so-called pale or white muscle: in others 



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