THE STRUCTURE OF THE ELEMENTARY TISSUES. 27 



Modes of Connection. Cells are connected together to form 

 tissues in various ways. 



(1.) By mean of a cementing intercellular substance. This is prob- 

 ably always present as a transparent, colorless, viscid, albuminous sub- 

 stance, even between the closely apposed cells of epithelium, while in 

 the case of cartilage it forms the main bulk of the tissue, and the cells 

 only appear as imbedded in, not as cemented together by, the intercel- 

 lular substance. This intercellular substance may be either homogene- 

 ous or fibrillated. In many cases (e.g., the cornea) it can be shown to 

 contain a number of irregular branched cavities, which communicate 

 with each other, and in which branched cells lie : through these branch- 

 ing spaces nutritive fluids can find their way into the very remotest 

 parts of a non-vascular tissue. 



As a special variety of intercellular substance must be mentioned the 

 basement membrane (mem brana propria) which is found at the base of 

 the epithelial cells in most mucous membranes, and especially as an 

 investing tunic of gland follicles which determines their shape, and 

 which may persist as a hyaline saccule after the gland cells have all 

 been discharged. 



(2.) By anastomosis of their processes. This is the usual way in 

 which stellate cells, e.g. of the cornea, are united : the individuality of 

 each cell is thus to a great extent lost by its connection with its neigh- 

 bors to form a reticulum : as an example of a network so produced we 

 may cite the stroma of lymphatic glands. 



Sometimes the branched processes breaking up into a maze of 

 minute fibrils, adjoining cells are connected by an intermediate reticu- 

 lum: this is the case in the nerve cells of the spinal cord. 



Derived Tissue-elements. Besides the Cell, which may be termed 

 the primary tissue-element, there are materials which may be termed 

 secondary or derived tissue-elements. Such are Intercellular substance, 

 Fibres, and Tubules. 



a. Intercellular substance is probably in all cases directly derived 

 from the cells themselves. In some cases (e.g. cartilage), by the use of 

 reagents the cementing intercellular substance is, as it were, analyzed 

 into various masses, each arranged in concentric layers around a cell or 

 group of cells, from which it was probably derived. 



/?. Fibres. In the case of the crystalline lens, and of muscle both 

 striated and non-striated, each fibre is simply a metamorphosed cell: in 

 the case of a striped fibre, the elongation being accompanied by a mul- 

 tiplication of the nuclei. The various fibres and fibrillae of connective 

 tissue result from a gradual transformation of an originally homogene- 

 ous intercellular substance. Fibres thus formed may undergo great 

 chemical as well as physical transformation : this is notably the case 

 with yellow elastic tissue, in which the sharply defined elastic fibres, 



