1046 



VEKTEBRATED ANIMALS 



FIG. 781. Cellular cartilage of 

 mouse's ear. 



in thin cells, so as to take off 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 

 external ear of a bat or mouse (fig. 

 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 

 coveringtheirsurface. Hence 

 they may be compared, in 

 regard to their grade of or- 

 ganisation, with the larger 

 alga3, which consist, like 

 them, of aggregations of cells 

 held together by intercellular 

 substance, without vessels of 

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

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

 from each other ; &, pair of cells in apposi- -, i /> mi 



tion; cc, nuclei of cartilage-cells; d, cavity whole surface. There are 

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

 behind). which the structureless inter- 



cellular substance is replaced 



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

 elastic ; such combinations, which are termed ^ro-cartilages, are 

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

 to be resisted ; as, for example, between the vertebrae of the spinal 

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

 of cartilage nothing more is necessary than to make very thin 



