CLASSIFICATION OF TISSUES. 29 



the others for their aid in turn, every necessary work is 

 better performed. And a need arises for a distributive 

 mechanism by which the excess products, if any, of various 

 tissues shall be carried to others which require them, and 

 for a regulative mechanism by which the activities of the 

 various tissues shall be rendered proportionate to the needs 

 of the whole Body at different times and under different 

 circumstances. Accordingly, as we may classify the in- 

 Iiabitantsof the United States into lawyers, doctors, clergy- 

 men, merchants, farmers, and so on, we may 



Classify tha Tissues, by selecting the most distinctive 

 properties of each of those entering into the construc- 

 tion of the adult Body and arranging them into physio- 

 logical groups; those of each group being characterized by 

 some one prominent employment. No such classification, 

 3io\vever, can be more than approximately accurate, since 

 the same tissue has often more than one well-marked 

 physiological property. The following arrangement, how- 

 ever, is practically convenient. 



1 . Undififerentiated Tissues. These are composed of 

 cells which have developed along no one special line, but 

 retain very much the form and properties of the cells form- 

 ing the very young Body before different tissues were re- 

 cognizable in it. The lymph corpuscle's and the colorless 

 ^corpuscles of the blood belong to this class. 



2. Supporting Tissues. Including* cartilage (gristle), 

 &one, and connective tissue. Of the latter there are several 

 subsidiary varieties, the two more important being white 

 fibrous connective tissue, composed mainly of colorless in- 

 extensible fibres, and yellow fibrous tissue, composed mainly 

 of yellow elastic fibres. All the supporting tissues are used 

 in the Body for mechanical purposes: the bones and carti- 

 lages form the hard framework by which softer tissues are 

 supported and protected; and the connective tissues unite 

 the various bones and cartilages, form investing mem- 

 -branes around different organs, and in the form of fine 

 networks penetrate their substance and support their con- 

 stituent cells. The functions of these tissues being for the 

 most part to passively resist strain or pressure, none of 



