178 THE CELL-THEORY 



found this common parent of all tissues itself to consist of 

 cells ; our next task must be to demonstrate not only in 

 this general way that tissues originate from cells, but also 

 that the special formative mass of each tissue is composed 

 of cells, and that all tissues are either constituted by simple 

 cells or by one or other of the manifold kinds of modified 

 cells" (p. 71). Five classes of tissue can be distinguished, 

 according to the extent and manner of the modifications 

 which the cells composing them have undergone. There are 

 first of all independent and isolated cells, such as the 

 corpuscles of the blood and lymph, not forming a coherent 

 tissue in the ordinary sense. Next there are the assemblages 

 of cells lying in contiguity with one another, but not in any 

 way fused ; examples of this class are the epidermal tissues 

 and the lens of the eye. In the third class come tissues 

 the cells of which have fused by their walls, but whose cell- 

 cavities are not in continuity, such as osseous tissue and 

 cartilage. In the tissues of the fourth class, comprising the 

 most highly specialised of all, not only are the cell-walls 

 continuous but also the cell-cavities ; to this class belong 

 muscle, nerve and capillary vessels. A fifth class, of 

 rather a special nature, includes the fibrous tissues of all 

 kinds. This is the first classification of tissues upon a 

 cellular basis, and it marks the foundation of a new histology 

 which took the place of the " general anatomy " of Bichat. 

 The exhaustive account which Schwann gives of the 

 structure and development of the tissues in this section of 

 his book constitutes the first systematic treatise on histology 

 in the modern sense, and it is still worth reading, in spite of 

 many errors in detail. 



Schwann found it easy to demonstrate the cellular nature 

 of the tissues of his first three classes. With the other two 

 classes he had more difficult}-. Fibres of all kinds, he 

 considered, arose by an elongation of cells, which afterwards 

 split longitudinally into long strips, forming as the case 

 might be white or elastic fibrous tissue. Muscle-fibres and 

 nerve-fibres were formed in a totally different way, by 

 coalescence of cells ; each separate muscle-fibre and nerve- 

 fibre was thus a compound cell. Capillaries, Schwann held, 

 were formed by cells hollowed out like drain-pipes, and 



