VERTEBRATED ANIMALS. 



279 



separated from each other by an t 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 different stages of the growth of any one. Thus 

 in the cartilage of the external ear of a bat or mouse (Fig. 469), the cells 

 are packed as closely together as are those of an ordinary Vegetable paren- 

 chyma (Fig. 236, A); and this seems to-be the early condition of most 

 cartilages that are afterwards to present a different aspect. In the ordi- 

 nary cartilages, however, that cover the extremities of the bones, so as to 

 form smooth surfaces for the working of the joints, the amount of inter- 

 cellular substance is usually considerable; and the cartilage-cells are com- 

 monly found imbedded there in clusters of two, three, or four (Fig. 470), 

 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 mem- 

 brane covering their surface. Hence they may be compared, in regard 



FIG. 470. 



FIG. 471. 



Section of the branchial Cartilage of Tad- 



Eole: a, group of four cells, separating 

 rom eaeh other ; 6, pair of cells in apposition ; 

 c, c, nuclei of cartilage-cells; d, cavity con- 

 taining three cells (the fourth probably 

 behind). 



Ultimate Follicles of Mam- 

 mary Gland, with their secret- 

 ing cells a, a, containing nuclei 

 6, 6. 



to their grade of organization, with the larger Algae; which consist, like 

 them, of aggregations of cells held together by intercellular substance, 

 without vessels of any kind, and are nourished by imbibition through 

 their whole surface. There are many cases, however, in which the struc- 

 tureless intercellular substance is replaced by bundles of fibres, sometimes 

 elastic, but more commonly non-elastic; such combinations, which are 

 termed /2#r0-cartilages, are interposed in certain joints, wherein tension 

 as well as pressure has to be resisted, as, for example, between the verte- 

 brae 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 

 sections with a sharp razor or scalpel, or, if the specimen be large and 

 dense (as the cartilage of the ribs), with the Microtome. These sections 

 maybe mounted in weak spirit, Goadby's solution, or glycerine- jelly; but 

 in whatever way they are mounted, they undergo a gradual change by 

 lapse of time, which renders them less fit to display the characteristic 

 features of their structure. 



676. Structure of the Glands. The various Secretions of the body (as 



