r^I^ THE CELL AND MAMMALIAN HISTOLOGY 



MECHANICAL TISSUE 



Mechanical tissues may contain amoebocytes as well as the 

 mechanocytes which are their fundamental constituents, but 

 their chief structural property is that they contain much inter- 

 cellular or ground substance, formed by, but external to, the cells 

 themselves. It may be in the form of fibres or plates, or may 

 be simply a homogeneous jelly or fluid, but whatever its form, 

 it is not itself hving. Although the best classification of mechanical 

 tissues would be by the types of cell, for it is on these that the 

 ground substance depends, it is convenient in practice to work 

 from the whole appearance of the tissues. This depends in great 

 part on the intercellular substance, and as this is also intimately 

 related to the function of the tissue, a classification so derived is 

 physiological as well as morphological. 



CONNECTIVE TISSUE 



The least specialised of mechanical tissues are the connective 

 tissues, the chief feature of which is the fibre. They serve not only 

 to connect one organ or tissue with another, but also, especially 

 where in their more highly developed form they make tough 

 sheets or fasciae, to separate them. The ordinary process of dis- 

 section of blood vessels and nerves consists in cutting and tearing 

 connective tissue, and it is only in the first opening of the body 

 and in some special circumstances that anything else should ever 

 be cut. The simplest connective tissue is areolar, which is well seen 

 in the white mesh work which in most places joins the skin to the 

 underlying body-wall (Fig. 400). There is a more or less continuous 

 jelly-like ground substance, which contains much chloride and 

 so stains black with silver nitrate, and in this run branching 

 and anastomosing bundles of white fibres. The material of which 

 these are made is the protein collagen, which on boiling forms 

 gelatin or glue (the difference between these two lies solely in 

 their degree of purity). They may be recognised in a preparation 

 by the facts that they show a wavy outline, and if broken do 

 not pull the cut ends apart, and because although the bundles 

 branch and meet again to form a network, individual fibres 

 never do. In areolar tissue there may also be a few yellow or 

 elastic fibres, which are made of another protein, called elastin, 

 which is not hydrolysed by boiling. The fibres run singly instead 



