THE STRUCTURE OF THE ELEMENTARY TISSUES. 4? 



Structure. Adenoid or retiform tissue consists of a very delicate 

 network of minute fibrils, formed originally by the union of processes 

 of branched connective-tissue corpuscles, the nuclei of which, however, 

 are visible only during the early periods of development of the tissue 

 (fig. 46). The network of fibrils is concealed by being covered with 

 flattened connective-tissue corpuscles, which may be readily dissolved 

 in caustic potash, leaving the network bare. The network consists of 

 white fibres, the interstices of which are filled with lymph corpuscles. 

 The cement substance of adenoid tissue is very fluid. 



Some authors make a distinction between retiform and adenoid tis- 

 sues, the former being the meshwork, and the latter the meshwork with 

 its contained lymph cells. 



Development of Fibrous Tissues. In the embryo the place of 

 the fibrous tissues is at first occupied by a mass of roundish cells, de- 

 rived from the " mesoblast." 



Fi. 47. Portion of submucous tissue of gravid uterus of sow. a, Branched cells, more or less 

 spindle-shaped; 6, bundles of connective tissue. (Klein.) 



These develop either into a network of branched cells or into groups 

 of fusiform cells (fig. 47). 



The cells are imbedded in a semi-fluid albuminous substance derived 

 either from the cells themselves or from the neighboring blood-vessels; 

 this afterward forms the cement substance. In it fibres are developed, 

 either by some of the cells becoming fibrils, the others remaining as con- 

 nective-tissue corpuscles, or by the fibrils being developed from the out- 

 side layers of the protoplasm of the cells, which grow up again to their 

 original size and remain imbedded among the fibres. The process gives 

 rise to fibres arranged in the one case in interlacing networks (areolar 

 tissue), in the other in parallel bundles (white fibrous tissue). In the 

 mature forms of purely fibrous tissue not only the remnants of the cell- 

 substance, but even the nuclei, may disappear. The embryonic tissue, 

 from which elastic fibres are developed, is composed of fusiform cells, 

 and a structureless intercellular substance by the gradual fibrillation of 

 which elastic fibres are formed. The fusiform cells dwindle in size and 

 eventually disappear so completely that in mature elastic tissue hardly 

 a trace of them is to be found : meanwhile the elastic fibres steadily in- 

 crease in size. 



