THEIR STRUCTURE, COMPOSITION, AND ACTIONS. 223 



218. In a purely anatomical classification, the order in which these Tissues 

 would be most appropriately arranged, would doubtless be that of their relation 

 to the primitive types already described; but such a classification, strictly 

 followed out, would involve so many physiological incongruities, as to render it 

 unsuitable to our present purpose. For the particular office which each tissue 

 performs in the vital economy, depends, not upon its own structure and endow- 

 ments alone, but upon its position in reference to that of others; and thus, if we 

 grouped together all the tissues consisting of unaltered cells, we should find a 

 certain set adapted to introduce nourishment into the blood from the contents 

 of the alimentary canal, in virtue of their position, and of their inherent power 

 of selection and appropriation; whilst another set, drawing a similar material 

 out of the blood, converts it into a portion of the solid fabric; and a third, by 

 the exercise of the very same powers, removes from the circulating fluid the 

 final products of the retrograde metamorphosis of the histogenetic substances, 

 and pours them back (it may be) into the very cavity from which those sub- 

 stances were originally drawn. If, on the other hand, we were to consider 

 these Tissues in their physiological aspect only, as the instruments of so many 

 distinct classes of operations, which all concur in the maintenance of the general 

 life of the organism, we might be led to attach too little importance to their 

 fundamental relations to each other and to the primitive forms out of which 

 they are developed. Hence it will be advisable, in this as in a former instance 

 (CHAP, ii.), to adopt a mixed classification, which may, so far as practicable, 

 serve both purposes; and the Primary Tissues of the Human Body will be 

 arranged, therefore, under the following heads. 



I. The Simple Fibrous Tissues, including the elementary forms known as 

 the "white" and "yellow" fibrous tissues, and the various combinations and 

 arrangements of these, which are known as Areolar or " connective" tissue, 

 Tendons, Ligaments, Aponeuroses, &c.; all serving purposes of a purely mecha- 

 nical nature. 



ii. The Fibro- Cellular Membranes, which are composite structures, made up 

 of textures formed by interwoven fibres, of simple basement-membrane covering 

 the surface of these, and of one or more layers of cells upon the free surface 

 of the basement-membrane ; such are the Skin investing the exterior of the 

 body, the Mucous Membranes which are prolonged from the skin through all its 

 open cavities, and the Serous and Synovial Membranes which line the closed 

 cavities. With the Skin it will obviously be proper to consider the "epidermic 

 appendages," namely, the Hair and Nails; whilst with the Mucous Membranes, 

 the Glandular apparatus is no less naturally connected. 



in. Those purely Cellular Tissues, which form part of the interior fabric; 

 of these, the Adipose and the Cartilaginous are the types. 



iv. The "sclerous" tissues, Bones and Teeth, which are composed of an 

 animal basis that is partly fibrous, partly cellular, consolidated by calcareous 

 deposit. 



V. The Tubular Tissues, which serve for the conveyance of liquids through 

 the other tissues; namely, the Bloodvessels and Absorbents. 



VI. The Muscular Tissue, which is especially distinguished by its contractile 

 power; one form of it being composed of elongated primitive cells, the other of 

 parent-cells elongated or coalesced into tubes, within which are aggregations of 

 minute secondary cells. 



vn. The Nervous Tissue, which, like the preceding, is rather distinguished 

 by its vital endowments than by the peculiarity of its organic structure ; for it 

 partly consists of simple cells, whilst another part of it is formed by cells elon- 

 gated or coalesced into tubes. 



