1046 



VERTEBRATED ANIMALS 



in thin cells, so as to take oft' the pressure of the glass cover from 

 their surface, which would cause the escape of the oil-particles. No 

 method of mounting (so far as the Author is aware) is successful in 

 causing these cells permanently to retain their contents. 



Cartilage. In the ordinary forms of cartilage, also, we have an 

 example of a tissue obviously composed of cells ; but these are com- 

 monly separated from each other by 

 an ' intercellular substance.' which is 

 so closely adherent to the outer walls 

 of the cells as not to be separable 

 from them. The thickness of this 

 substance differs greatly in different 

 kinds of cartilage, and even in dif- 

 ferent stages of the growth of any 

 one. Thus in the cartilage of the 

 FIG. 781. Cellular cartilage of external ear of a bat or mouse (fig. 



mouse s ear. 



781), the cells are packed as closely 

 together as are those of an ordinary- 

 vegetable parenchyma; and this seems to be the early condition 

 of most cartilages that are afterwards to present a different 

 aspect. In the ordinary cartilages, however, that cover the ex- 

 tremities of the bones, so as to form smooth surfaces for the work- 

 ing of the joints, the amount of intercellular substance is usually 

 considerable ; and the cartilage-cells are commonly found imbedded 

 there in clusters of two, three, or four (fig. 782), which are evidently 

 formed by a process of ' binary subdivision.' The substance of these 



cellular cartilages is entirely 

 destitute of blood-vessels, 

 being nourished solely by 

 imbibition from the blood 

 brought to the membrane 

 covering their surface. Hence 

 they may be compar-ed, in 

 regard to their grade of or- 

 -anisation, with the larger 

 alga?, which consist, like 

 them, of aggregations of cells 

 held togei her by intercellular 

 substance, without vessels of 

 f FIG. 782. Section of the branchial cartilage of any kind, and are nourished 

 tadpole: a, group of four cells, separating bv imbibition through their 

 from each other; b, pair of cells in apposi- ', , rpi 



tion ;<-,-, nuclei of cartilage-cells; rf, cavity wh le Surface. There are 

 containing tluve cells (the fourth probably many cases, however, in 

 '"' llind )- which t he st met ureless inter- 



cellular substance is replaced 



by bundles of fibres, sometimes elastic, but more commonly 11011- 

 elastic; such combinations, which are termed ///'/-"-cartilages, are 

 interposed in certain joints, wherein tension as well as pressure has 

 to be resisted ; as. tor example, between the vertebra 1 of the spinal 

 column and the bones of the pelvis. In examining the structure 

 of cartilage nothing more is necessarv than to make verv thin 



