44 CONNECTIVE TISSUES. [sect. 22. 



Very frequently present the peculiarity of excreting secondary 

 membranes, and thus become transformed into thick-walled ve- 

 sicles (cells in the true and reticulated cartilages, cartilage-cells in 

 connective tissues), which are even capable of being metamorphosed 

 similarly to the lignified and dotted vegetable cells (cartilage-cells 

 in rachitic bones). When connective substance ossifies, the cells 

 mentioned pass into different forms, as round, stellate, or elongated, 

 as into the lacuna and canaliculi of bone, or the tubules of the 

 teeth, and then serve to transport the nourishing fluid into these 

 structures; a function which, moreover, they possess in many con- 

 nective txssues and in the cartilages, except that here they are not 

 always so favourably constituted for this purpose. In connective 

 tissue these cells frequently contain pigment, and all pigment-cells 

 of connective substance belong to this category. But while all 

 these cells, viz., those of mucous tissue and cartilage, colourless and 

 pigmentous corpuscles of connective tissue, bone-cells, and dental 

 canals, thus agree in all essential points, there occur, nevertheless, 

 in certain connective -tissue-corpuscles, further peculiar metamor- 

 phoses, by which they are ultimately converted into elastic fibres, 

 between which and the connective-substance-cells of other tissues 

 no points of comparison are any longer discoverable. With regard 

 to the chemical nature of the cells of connective substance, this 

 much is certain, that their membranes consist originally of a protein 

 compound, but, in the course of development, are frequently con- 

 verted into a substance very closely allied to that of elastic tissue 

 whose fibres, indeed, arise from such cells, Hence it arises that 

 m most connective substances, the cells can be easily isolated on 

 softening the matrix by boiling, or by maceration in acids or caustic 

 alkalies. It is only in certain cases that the membranes of con- 

 nective-substance-cells seem to yield gelatin, viz., in the external 

 secondary membrane of cartilage-cells at the period of ossification, 

 and, as it also appears, the cartilage-capsules in reticular cartilage. 

 Besides the above large and important group of cells in °the 

 tissue of connective substance, others occur, many of which, how- 

 ever, appear in such large proportions and apparent independence of 

 the connective substance, that it is impossible to place them in a 

 line with the proper cells of that substance. In this category I 

 place fat-cells of all kinds, the cells of parenchymatous organs not 

 referable to epithelial formations, such as the small cells in the 

 red medulla of bone, those of the shut follicles of the intestine 

 and spleen, the parenchymatous cells of the latter organ, and of 

 the supra-renal capsules, the ccUs of the lymphatic glands; lastly 



