58 MATERIAL COMPOSITION OF MAN. 



2dly. Mucous membranes line all the outlets of the body, the air- 

 passages, alimentary canal, urinary and genital organs, &c. 



3dly. Fibrous membranes form tendon, aponeurosis, ligament, &c. 



Compound membranes are formed by the union of the simple, and 

 are divided into fibro-serous, as the pericardium ; sero-mucous, as the 

 gall-bladder, at its lower part; and fibro-mucous, as the ureter. 



11. Areolar, cellular or laminated tissue to be described presently 

 is a sort of spongy or areolar structure, which forms the framework 

 of the solids; fills up the spaces between them, and serves at once as a 

 bond of union and separation. 



12. A viscus is the most complex solid of the body ; not only as re- 

 gards intimate organization, but use. This name is given to organs con- 

 tained in the splanchnic cavities, brain, thorax, and abdomen, and 

 hence the viscera are termed cerebral, thoracic, and abdominal. 



Every animal solid is either amorphous or fibrous; that is, it is either 

 without apparent arrangement, like jelly ; or is disposed in minute 

 threads, called fibres. The disposition of these threads, in different 

 structures, is various. Sometimes, they retain the form of threads ; at 

 others, they have that of laminae, lamellae, or plates. Accordingly, 

 when we examine any animal solid, where the organization is percep- 

 tible, it is found to be either amorphous, or fibrous and laminated. 



This circumstance led the ancients to endeavour to discover an ele- 

 mentary fibre or filament, from which the various organs might be formed. 

 Haller 1 embraced the idea, and endeavoured to unravel every texture 

 to this ultimate element, which, he conceived, is to the physiologist 

 what the line is to the geometer; and, as all figures can be constructed 

 from the line, so every tissue and organ of the body may be built up 

 from the filament. Haller, however, admitted that this elementary 

 fibre is not capable of demonstration, and that it is visible only to the 

 " mind's eye," " invisibilis ea fibra, quam sold mentis acie adtingi- 

 mus." It must be regarded, indeed, as a pure abstraction; for, as 

 different animal substances in the mass have different proportions of 

 carbon, hydrogen, oxygen, and nitrogen, it is fair to conclude that the 

 elementary fibre must equally differ in the different substances. 



The ancients believed that the first product of the elementary fibre 

 was areolar tissue ; and that this tissue forms every organ of the 

 body, the difference in the appearance of the organs arising from 

 the different degrees, of condensation of its laminae. Anatomists, 

 however, have been unable to reduce all animal solids to areolar tissue 

 only. 



In the upper classes of animals, three primary fibres or tissues or 

 anatomical elements are usually admitted, the areolar, cellular or 

 laminated; the muscular; and the nervous, pulpy or medullary. 



1. The areolar, cellular, mucous, filamentous or laminated fibre or 

 tissue is the most simple and abundant of animal solids. It exists in 

 every organized being ; and is an element of every solid. In the ena- 

 mel of the teeth only it has not been detected. It is formed of an 

 assemblage of thin laminae, of delicate, whitish, extensible filaments, 



1 Elementa Physiologic, vol. i. lib. i. sect. i. p. 7, Lausan., 1757. 



