CHAP. I.] THE TISSUES. 47 



The first texture enumerated in this table is an example of the 

 simplest form of membrane. Its principal character is extension ; 

 but as to the arrangement of its ultimate particles nothing is 

 known ; for, under the highest powers in the microscope it appears 

 homogeneous, that is, without visible limits to its particles, or, 

 at most, irregularly and very indistinctly granular. The capsule 

 of the lens, the posterior layer of the cornea, and the walls of the 

 primary organic cells, are composed of it ; and it is employed in 

 forming muscle, nerve, and the adipose and tegumentary tissues. 



The filamentous tissues are extensively used for connecting 

 different parts, or for associating the elements of other tissues. 

 The ligaments of joints, for instance, are composed of the white or 

 yellow fibrous tissues ; and areolar tissue surrounds and connects 

 the elementary parts of nerves and muscles, accompanies and 

 supports the blood-vessels, and unites the tegumentary tissues to 

 their subjacent parts or organs. 



Under the title compound membranes we include those expan- 

 sions, which form the external integument of the body, and are 

 continued into the various internal passages, which, by their invo- 

 lutions, contribute to form the various secreting organs or glands. 

 These are composed of the simple homogeneous membrane, covered 

 by epidermis or epithelium, and resting upon a layer of vessels, 

 nerves, and areolar tissue in great variety ; and they constitute the 

 skin, and mucous membranes, with the various glandular organs 

 which open upon their surface. Hairs and nails, being hardened 

 cuticle, are justly regarded as appendages to the former. 



To these, we may add those remarkable membranes, composed of 

 areolar tissue and a thin indusium of epithelium, which are em- 

 ployed as mechanical aids to motion. These are the serous mem- 

 branes which line the great cavities of the body, and the synovial 

 membranes, which are interposed between the articular extremities 

 of the bones in certain joints, or are connected with and facilitate 

 the motions of tendons. 



The tissues which compose the fourth class have no common 

 character, except their adherence, in the adult state, to the primitive 

 cellular structure, and their analogy in that particular with the 

 vegetable tissue. Although a certain agreement, in morphological 

 characters, allows these textures to be grouped together, none can 

 be more dissimilar as regards their vital endowments. They differ 

 materially as to the degree of cohesion between their cells : in 

 cartilage there is generally a firm and resisting intercellular sub- 

 stance ; in adipose tissue, the interval between the cells is occupied 



