HISTOLOGY. 21 
sue, which arises from a modification of the mesothelium. Except in 
the case of the muscles of the heart, the striped tissue is under control of 
the will; it usually occurs in larger masses than does the smooth, and 
is capable of rapid contraction. It differs structurally from smooth 
muscle. Instead of distinct, uninucleate cells there are long cylindrical 
elements (fig. 13, B), the primitive fibres, each with several nuclei in the 
interior in lower vertebrates, on its periphery in the higher. Most of 
the protoplasm of the fibre has been altered to minute contractile 
fibrille, each crossed by lighter and darker bands, and as these come 
opposite each other in the different fibrille, they give the fibre its 
characteristic cross-banded appearance. 
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Fic. 13.—A, smooth muscle cell; B, striped muscle. 
The primitive fibres rarely branch at their extremities. Each is 
surrounded by a structureless envelope, the sarcolemma, while num- 
bers of fibres are bound into bundles and muscles by connective 
tissue (perimysium) which carries nerves and blood-vessels. At the 
ends of the bundles the perimysium continues into the tendons which 
attach the muscles to other parts. 
The heart muscle also arises from the mesothelium, is cross-banded, 
but is removed from control of the will. The cells are usually short 
(usually with a single nucleus); they branch, the branches connecting 
adjacent muscle cells. 
Connective Tissues. 
The tissues grouped here arise from the mesenchyme and are 
distinguished from all other tissues by the great amount of intercellular 
substance produced by the cells themselves. This substance or matrix 
varies in character and determines the variety of tissue. Frequently it 
is dense and hence the connective tissues may give the body support, 
and in fact they are sometimes called supportive tissues. 
