102 



MANUAL OF HISTOLOGY. 



Fig. 109. Human mus- 

 cle-fibre undergoing 

 fatty degeneration. 



careous matters in cells, may at least be partially regarded as physiological 

 processes, but belong probably, in the elementary parts with which we are 

 now engaged (like many other modes of degeneration), 

 in a great measure to the pathological changes of the 

 system. Later on we shall be obliged to enter more 

 fully into the consideration of this subject. 



64. 



Now, by the combination of structural- elements 

 of similar or dissimilar kinds, and in larger or smaller 

 quantity, the various tissues of the human and animal 

 body generally are formed. These are naturally regu- 

 lated as regards their anatomical texture, chemical 

 constitution, and physiological properties, by the ele- 

 mentary parts of which they are composed. 



A classification of tissues that shall have any 

 scientific value is still a matter of the greatest dif- 

 ficulty nay, we might almost say, of impossibility. 

 Such a classification, namely, can be founded only on 

 a knowledge of the mode of developmen^ of the 

 structural elements. But histogenesis, unfortunately, although com- 

 manding a considerable amount of material in many branches of our 

 science, is yet but very imperfect in others. The history of the origin 

 of tissues is, as a whole, not far enough advanced to enable us accu- 

 rately, and without being obliged to resort to many hypotheses, to 

 trace the outlines of a scientific classification of the various tissues. Even 

 that apparently easy and accurate division into simple and composite 

 textures cannot be strictly adhered to, and the question whether we 

 have before us a composite tissue or not must, in many cases, be decided 

 according to individual opinion, as to whether certain metamorphosed 

 portions of the ground-substances are to be considered as structural 

 elements or no. 



The following classification, therefore, is only to be accepted as pro- 

 visional, being designed (as is usually the case in artificial systems) more 

 to bring in review in a certain order the materials to be considered, 

 than always rigidly to associate together parts probably related to one 

 another in their mode of development. The practical objects aimed at 

 in this work will render it necessary, besides, to consider many things 

 together, which logically should be dealt with separately. The following 

 is our division : 



A. Tissues composed of simple cells with fluid intercellular sub- 



stance. 



1. Blood. 



2. Lymph and Chyle. 



B. Tissues composed of simple cells with a small amount of solid 



intercellular substance. 



3. Epithelium. 



4. Nail. 



