FORMATION OF THE TISSUES. PIGMENT-CELLS. 467 



it is soluble in 4 parts of water, and is taken up in small quantity by alcohol. 

 This fact is one of much interest, in regard to certain Pathological relations of 

 Gelatin. 



[The following table presents, perhaps, the best general view of the various tissues. 

 No satisfactory arrangement can be constructed, based on any one principle of classifi- 

 cation. 



TABULAR VIEW OF THE TISSUES OF THE HUMAN BODY. 



1. Simple membrane, homogeneous, or nearly so, Examples. Posterior layer of the 



employed alone, or in the formation of com- cornea. Capsule of the lens. 



pound membranes. Sarcolemma of muscle, &c. 



2. Filamentous tissues, the elements of which are White and yellow fibrous tissues. 



real or apparent filaments. Areolar tissue. 



3. Compound membranes, composed of simple Mucous membrane. Skin. True or 



membrane, and a layer of cells, of various secreting glands. Serous and sy- 



forms (epithelium or epidermis), or of areo- novial membranes, 

 lar tissue and epithelium. 



4. Tissues which retain the primitive cellular Adipose tissue. Cartilage. Gray 



structure as their permanent character. nervous matter. 



5. Sclerous or hard tissue. Bone. Teeth. 



6. Compound tissues. 



a. Composed of tubes of homogeneous mem- Muscle. Nerve, 

 brane, containing a peculiar substance. 



b. Composed of white fibrous tissues and Fibro-cartilage. 

 cartilage. M. C.j 



616. There are several instances in which Nucleated Cells, resembling those 

 of the primordial fabric, are seen even in the adult body. The most striking 

 examples of this are to be found among the inferior members of the class of 

 Fishes. Thus, in the Myxinoid family, there is no true Vertebral column, 

 but its place is occupied by a gelatinous tube, termed the chorda dorsalis ; 

 which consists entirely of nucleated cellular tissue, and which is precisely 

 analogous to the structure occupying the same situation in the early Embryo 

 of higher animals ( 760). In the Short Sun-fish, a corresponding form of 

 tissue forms a thick covering to the body, replacing the true skin. And in the 

 Lancelot (a little fish which is deficient in so many of the characters of the 

 Vertebrated division, that many naturalists have doubted its right to a place in 

 the class), a considerable portion of the fabric is made up of a similar paren- 

 chyma. We shall find, however, that even in Man a considerable part of the 

 fabric is made up of Cells ; and that these perform some of the most important 

 offices in the economy. 



617. The Pigment-cells, which give colour to the Skin, and of which the 

 Pigmentum Nigrum of the eye is entirely composed, usually exhibit the ori- 

 ginal form of the cell with little alteration. On the choroid coat of the eye 

 they are seen as a kind of pavement, having somewhat of a polyhedral shape, 

 and lying in a very regular manner with some intercellular substance interposed 

 between them. In the Skin of Man, they are scattered through the ordinary 

 epidermic cells ; and its colour is determined by that of their contents. There 

 is no distinct coloured layer, as was formerly supposed ; but the cells are more 

 closely aggregated in some parts than in others. This is as much the case in 

 the European, however, as in the Negro ; in the former, they are concerned in 

 producing the spots termed freckles, and others of a similar kind. In some 

 animals, the Pigment cells of the skin frequently undergo a change of form ; 

 being elongated in many directions into hollow fibres, which, meeting other 

 formations of the same kind, produce a more or less perfect network of star- 

 shaped cells. This change is best seen in the skin of the Batrachia, where 

 the cells are frequently isolated : a good example of it is shown in Fig. 89 (p. 

 360.) The black colour is given by an accumulation within the cell, of a 



