CLASSIFICATION OF HUMAN ELEMENTARY TISSUES. 119 



belief in the production of the Areolar and other fibrous tissues by Cell-trans- 

 formation. An additional argument in favour of this view, may be found in 

 the appearances presented by the semi-fibrous Cartilages. In the Cartilages 

 of the ribs, for instance, a more or less distinct fibrous appearance may often 

 be seen in the intercellular substance, which is elsewhere quite homogene- 

 ous ; this appearance is sometimes so faint, that it might be considered as 

 an illusion, occasioned by the manipulation to which the section has been 

 subjected; but it is often so well defined, as to present the aspect of true 

 fibrous tissue. No indication of the direct operation of cells, in the develop- 

 ment of these fibres, has ever been witnessed ; and we can scarcely do other- 

 wise than regard them as produced by the regular arrangement and con- 

 solidation of the particles of the intercellular substance, in virtue of its own 

 inherent powers. 



137. The following arrangement of the Human Tissues will be here adopt- 

 ed as expressing their respective relations to the fundamental elements which 

 have been now described; namely, simple Membrane, Fibres, and Cells. 



a. Simple Membranous Tissues. Of these there are scarcely any examples 

 in the Human body, except in the posterior layer of the cornea and the cap- 

 sule of the crystalline lens. The membranous element is largely found, how- 

 ever, in the compound Membrano-fibrous tissues. 



b. Simple Fibrous Tissues. Under this head may be classed the White 

 and Yellow Fibrous Tissues, and Areolar Tissue. 



c. Simple Cells floating separately and freely in the fluids. Such are the 

 Corpuscles of the Blood, Chyle, and Lymph. 



d. Simple Cells developed on the free surfaces of the body. Such are the 

 Epidermis and Epithelium. 



e. Compound Membrano-Fibrous Tissues, composed of a layer of simple 

 membrane, developing Cells on its free surface, and united on the other to a 

 fibrous or areolar structure. Of this kind are the Skin, the Mucous Mem- 

 branes, the Serous and Synovial membranes, the lining membranes of the 

 Blood-vessels, &c. 



f. Simple Isolated Cells, forming solid tissues by their aggregation. Un- 

 der this head we may rank the Fat-cells, the Vesicles of Gray Nervous matter,* 

 the Absorbent cells at the extremities of the Intestinal villi, and the cellular 

 parenchyma of the Spleen and similar bodies : the cells being held together, 

 in all these cases, by the blood-vessels and areolar tissue which pass in amongst 

 them. In Cartilage, and certain tissues allied to it in structure, the cells are 

 united by intercellular substance, which may be quite homogeneous, or may 

 have a fibrous character. 



g. Sclerous or Hard Tissues, in which the cells have been consolidated by 

 internal deposit, and have more or less completely coalesced with each other. 

 Such is the case with the substance of Hair, Nails, &c., which may be more 

 properly ranked under the Epidemic Tissues ; but the result is most charac- 

 teristically seen in Bones and Teeth. 



h. Simple Tubular Tissues, formed by the coalescence of the cavities of 

 cells, without secondary internal deposit. The Capillary blood-vessels, and 

 probably also the smallest Lymphatics and Lacteals, seem to be formed in this 

 manner. 



i. Compound Tubular Tissues ; in which, subsequently to the coalescence 

 of the original cells, a new deposit has taken place within their cavities. In 

 the ttobuli of the White or Medullary Nervous matter, and in those of the least 



* As it is undesirable to separate from each other the descriptions of the two elementary 

 forms of Nervous structure, on account of their close functional connection, the gray or vesi- 

 cular nervous matter will be described together with the white or tubular, in the last section 

 of this chapter. 



