22 PHYSIOLOGY. 



the elements of the food contribute icnmeJiately to their formation. 

 Of this latter hypothesis the bile is supposed to be an example. 



Further, in regard to the particular tissues from which these com- 

 pounds are formed, Dr. Prout suggests that urea is derived from the 

 gelatinous, and uric acid from the albuminous tissues, whilst the 

 fatty materials are supposed to form some of the constituents of the 

 bile. The particular compounds will be treated of under the head 

 of secretions. 



CLASSIFICATION OF THE TISSUES. 



From the proximate principles described above, are developed the 

 various tissues of the body. In combining to form the different 

 structures the solids are arranged in a variety of ways. Of these the 

 chief are in filaments, or elementary fibres, tissues, organs, apparatus, 

 and systems, ^y filament^ is meant the elementary solid, k fibre 

 consists of a number of filaments united together. By the union of 

 tissues, organs are formed. A number of difftu'ent organs united to 

 accomplish one end constitute an apparatus. When a number of 

 organs of similar or analogous structure are united for one end, they 

 form a system. Schwann advanced the doctrine that all the tissues 

 of the body were formed from cells. Tt has been shown, however, 

 by subsequent research, that this assertion was rather too hasty ; that 

 although many tissues retain their original cellular type throughout 

 life, and many more are formed from cells which are afterwards 

 metamorphosed, there are some in which no other cell-agency is 

 employed than that concerned in the elaboration of the plastic ma- 

 terial. This is the case in certain forms of the very delicate struc- 

 tureless lamella known by the name of basement ^netnbrane^ found 

 beneath the epidermis and epithelium, in which no vestige of cell 

 structure can be seen, but which appears rather to resemble that of 

 which the cell walls are themselves constituted. At other times it 

 seems to be composed of the coalesced nuclei of cells whose deve- 

 lopment has been arrested. In regard to the fibrous tissues a doubt 

 also exists as to whether they are developed by a metamorphosis of 

 cells, or whether they are not, like ba'^ement membrane, produced 

 by a consolidation of a plastic fluid which has been elaborated by 

 cells. 



The following arrangement of the human tissues, is that adopted 

 by Dr. Carpenter, and expresses their fundamental relation to the 

 elements above alluded to, viz. : membrane, fihres, and cells. 



a. Simple membranous tissues. — Of these there are scarcely any 

 examples in the human body except in the capsule of the lens, and 

 the posterior layer of the cornea. The walls of the primary organic 

 cells are also composed of it ; and it is employed in forming muscle, 

 nerve, and the adipose and tegumontary tissues. Its principal cha- 



