CLASSIFICATION OF THE TISSUES. 23 



racter is extension, but its ultimate arrangement defies the highest 

 powers of the microscope. 



h. Simple fibrous tissues. — Under this head are included the 

 white and yellow fibrous tissue and the areolar tissue. These are 

 extensively used for connecting different parts, or for associating the 

 elements of other tissues. The ligaments of joints are composed of 

 the white, or yellow fibrous tissues ; and areolar tissue surrounds 

 and connects the component parts of nerves, muscles, vessels, &c. 



c. Simpk cells floating separately and freely in the fluids, as 

 corpuscles of the blood, lymph, and chyle. 



d. Simple cells developed on the free surfaces of the body, as 

 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, as the skin, mucous mem- 

 branes, serous and synovial membranes, lining membrane of blood- 

 vessels, &c. 



/ Simple isolated cells, forming solid tissues by their aggregation, 

 as fat cells, the vesicles of gray nervous matter, absorbent cells of 

 the villi, the cellular parenchyma of the spleen ; the cells being held 

 together in all these cases by the blood-.vessels and areolar tissue 

 which pass in between them. In cartilage, and certain tissues allied 

 to it in structure, the cells are united by intercellular substance, 

 either homogeneous, or of a fibrous character. 



g. Sclerous or hard tissues, in which the cells have been more 

 or less consolidated by internal deposit, and more or less completely 

 coalesced with each other, as the hair, nails, &c. These may be 

 more properly ranked under the epidermic tissues, but the result is 

 more characteristically seen in bones and teeth. The sclerous tissue 

 contains a large proportion of inorganic material to which it owes 

 its hardness. It differs from all the other tissues except cartilage 

 and fibro-cartilage, which for hardness might be classed with it. 



h. Simple tubular tissues, formed by the coalescence of the cavi- 

 ties of cells, without secondary internal deposit, as the capillary 

 blood-vessels and smallest lacteals and lymphatics. 



i. Compound tubular tissues, in which, subsequently to the coa- 

 lescence of the original cells, a new deposit has taken place within 

 their cavities. In the tubuli of the white nervous matter, and in 

 those of the least perfect form of muscular fibre, the secondary de- 

 posit has only a granular or amorphous character ; but in the 

 striated muscular fibre it is composed of minute cells. 



Other classifications of the tissues have^been adopted by different 

 physiologists and general anatomists, among these Haller described 

 only ^^ree,the nervous^ the muscular^ and the cellular ; from these all 

 the organs are formed by the association of the first two tissues, or by 

 the last alone. The Hmits of a work like this will not admit of an 



