CELLS CONNECTED TOGETHER IN SOLID TISSUES. 155 



t take so active a part in its vital operations. These we shall find 

 to be usually more or less closely connected together, either by a general 

 enveloping membrane, or by an intercellular substance, which is inter- 

 posed between their walls, and holds them together by its adhesive 

 properties. 



249. The presence of a general enveloping membrane (where it is not 

 a secondary formation) appears to depend upon the persistence of the 

 original cell-walls ; which, instead of liquefying or thinning away, when 

 distended by the multiplication of cells in their interior, are thickened or 

 strengthened by additional nutrition. Such is perhaps the case with the 

 sacculi in which the cells of Adipose tissue ( 257) are often found clustered 

 together ; but this condition is usually much more obvious in many 

 tumours, whose development depends upon an abnormal process of growth. 



250. Where such enveloping membranes are wanting, we frequently 

 find the component cells of the permanent tissues of Animals (like those 

 of the higher plants) held together by an intercellular substance ; which 

 generally presents no distinct traces of organization ; and which usually 

 consists of Gelatine, or of a substance allied to it in composition. The 

 proportion of this substance to the cells may vary in different cases ; and 

 very different characters may thus be presented by a tissue made up of 

 the same elements. Thus the subjoined figure (33) represents a portion 

 of one of the animal layers included between the calcareous laminae of a 

 bivalve shell; in which we see on the one side a number of nuclei or 

 incipient cells, scattered through a bed of homogeneous intercellular sub- 

 stance, and bearing but a very small proportion to it ; whilst the opposite 

 end exhibits a set of polygonal cells, in close contact with each other, 

 the intercellular substance being only represented by the thick dark 

 lines, which mark the boundaries of the cells, and which are rather 

 thicker at the angles of the latter. Between these two extremes, we 

 observe every stage of transition. 



251. The presence of a very large amount of intercellular substance, 

 through which minute cells are scattered at considerable intervals (Fig. 

 B3, a), is characteristic of various forms of Cartilage ; and more par- 

 ticularly of that soft semi-cartilaginous structure, of which the Jelly-fish 

 are for the most part composed. In other forms of cartilage, we find 

 the cells more developed, and in closer proximity to each other, the 

 proportion of the intercellular substance being at the same time dimi- 

 nished (as seen at b and c, Fig. 33) ; but it is not often, save in the 

 embryonic structures, that we find the cells in such, close proximity, and 

 the intercellular substance so nearly wanting, as at d. Such examples 

 do occasionally present themselves, however, even in the soft tissues. 

 Thus the chorda dorsalis, which replaces the vertebral column in the 

 lowest Fishes, and of which the analogue is found in the embryos of the 

 higher Vertebrata, is made up of a structure of this kind (Fig. 16), 

 The true Skin in the Short Sun-fish, is replaced by a similar layer of 

 cellular tissue, which extends over the whole body, varying in thickness 

 from one-fourth of an inch to six inches. And in the Lancelot (a little 

 fish which is destitute of so many of the characters of a Vertebrated 

 animal, that its right to a place in that division has been doubted), a 

 considerable portion of the fabric is made up of a similar parenchyma. 



