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 

 fibrillae, each crossed by lighter and darker bands, and as these come 

 opposite each other in the different fibrillae, they give the fibre its 

 characteristic cross-banded appearance. 



^~^.^_^= 



FIG. 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. 



