THE HUMAN BODY. 



nore than approximately accurate, since the same tissue has 

 often more than one well-marked physiological property. 

 The following arrangement, however, is practically convenient 



1. UNDIFFERENTIATED TISSUES. These are composed oil 

 cells which have developed along no one special line, but 

 retain very 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 oJ 

 the blood belong to this class. 



2. SUPPUKIJNG TISSUES. Including cartilage (gristle), 

 and connective tissue. Of the latter there are seven 



subsidiary varieties, the two more important being -whiti 

 fibrous connective tissue, composed mainly of colorless inex- 

 tensible fibres, and yelloio fibrous tissue, composed mainly of 

 yellow elastic fibres. All the supporting tissues are used in 

 the Body for mechanical purposes : the bones and cartilage 

 form the hard framework by which softer tissues are supporte 

 and protected; and the connective tissues unite the various 

 bones and cartilages, form investing membranes around dif- 

 ferent 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 to passively resist 

 strain or pressure, none of them has any very marked phy- 

 siological property; they are not, for exai ( ^ .\ fri-i table or 

 contractile, ;iiid their mass is chiefly made up o'i an intercell- 

 ular substance which has been formed by the actively living 

 cells sparsely scattered through them, as for instance in 

 cartilage, Fig. 45, where the cells are seen imbedded in cavi- 

 ties in a matrix which they have formed around them; and 

 this matrix by its firmness and elasticity forms the func- 

 tionally important part of the tissue. * 



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

 members of which fall into three main divisions, viz. : 



[ssimilative tissues, concerned in receiving and preparing 

 food materials, and including (a) Secretory tissues, com- 

 posed 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 feretory tissues, represented by cells in the 



