TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS 31 



much the form and properties of the cells forming the very young" 

 Body before different tissues were recognizable in it. The lymph- 

 corpuscles and the colorless corpuscles of the blood belong to this 

 class. 



2. SUPPORTING TISSUES. Including cartilage (gristle), bone, and 

 connective tissue. Of the latter there are several subsidiary vari- 

 eties, the two more important being white fibrous connective tissue, 

 composed mainly of colorless inextensible fibers, and yellow fibrous 

 tissue, composed mainly of yellow elastic fibers. All the support- 

 ing tissues are used in the Body for mechanical purposes; the bones 

 and cartilages form the hard framework by which softer tissues are 

 supported and protected; and the connective tissues unite the 

 various bones and cartilages, form investing membranes around 

 different organs, and in the form of fine networks penetrate their 

 substance and support their constituent cells. The functions of 

 these tissues being for the most part passively to resist strain or 

 pressure, none of them has any very marked physiological prop- 

 erty; they are not, for example, excitable or contractile, and their 

 mass is chiefly made up of an intercellular substance which has 

 been formed by the actively living cells sparsely scattered through 

 them, as, for instance, in cartilage (Fig. 11), where the cells are seen 

 imbedded in cavities in a matrix which they have formed around 

 them; and this matrix by its firmness and elasticity forms the 

 functionally important part of the tissue. 



3. NUTRITIVE TISSUES. These form a large group, the members 

 of which fall into three main divisions, viz. : 



Assimilative tissues, concerned in receiving and preparing food 

 materials, and including (a) Secretory tissues, composed of cells 

 which make the digestive liquids poured into the alimentary canal 

 and used to bring about chemical or other changes in the food. 

 (b) Receptive tissues, represented by cells which line parts of the 

 alimentary canal and take up the digested food. 



Eliminative or excretory tissues, represented by cells in the kid- 

 neys, skin, and elsewhere, whose main business it is to get rid of the 

 waste products of the various parts of the Body. 



Respiratory tissues. These are concerned in the gaseous inter- 

 changes between the Body and the surrounding air. They are 

 constituted by the cells lining the lungs and by the colored cor- 

 puscles of the blood. 



