26 



INTRODUCTION 



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 usu- 

 ally short (usually with a single nucleus) ; they branch, the branches 

 connecting adjacent muscle cells. 



Fig. 1 6. — A, smooth muscle cell; B, striped muscle. 



Connective Tissues 



The tissues grouped here arise from the mesenchyme and are dis- 

 tinguished 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. 



In the earliest phase, known as embryonic comiective tissue 

 (fig. 17, A), the cells are scattered, with long radiating processes, and 

 between the cells is a thin gelatinous matter. It is by increase of this 

 intercellular substance by taking up water that many embryos gain 

 so in size without taking food. The embryonic connective tissue 

 may develop in various directions. 



Thus some of the cells may contain pigment granules, forming 

 pigment cells (B), or oil globules may be deposited in them to such an 

 extent that the cells become spherical, while the intercellular sub- 

 stance is reduced, thus affording fat or adipose tissue. Most com- 

 mon of the connective tissues is fibrous tissue (white or non-elastic 

 tissue) in which the cells are branched or spindle-shaped while the 

 matrix is filled with fine fibrillae of considerable strength and little 

 elasticity. 



