Metasoa Ran a. 297 



front of the rectal aperture. From the bladder the urine 

 escapes to the exterior periodically. 



So far we have followed the course of the blood only to 

 and from special organs. A very large amount of the blood 

 is devoted, however, to the nourishing and oxygenating of 

 the muscular and skeletal or supporting system. At p. 273 

 we have glanced at the histological characters of muscle, so 

 that we may now proceed to the consideration of the hard 

 parts or, in other words, the skeleton, or skeletal system. 



The only representative of the skeletal system we have 

 yet had to deal with was the notochord of Amphioxus, a 

 structure, however, of supreme importance ; for, as indicated 

 at p. 258, we find that this rod forms the basis on which 

 the chief part of the skeleton of the frog and of higher 

 animals as well is built. 



The skeleton is composed of two substances, bone and 

 cartilage. The histological character of these are of some 

 importance. 



Cartilage is, like most other tissues, composed of a 

 matrix in which are embedded numerous cells. The matrix 

 is relatively very abundant ; it consists of a homogeneous 

 intercellular substance very elastic in its nature. The cells 

 are spherical or polygonal, and frequently occur in small 

 groups which result from the fissiparous division of a single 

 cell. By studying the process of division it may be made 

 out that the matrix is produced by modification of the old 

 cell- walls, which are successively cast off as new walls are 

 formed round the daughter-cells. 



The matrix is not always homogeneous. In one variety 

 of cartilage there may be formed in the matrix delicate 

 elastic fibrils (yellow fibro-cartilage), in another wavy fibres 

 of white fibrous tissue (white fibro-cartilage) (fig. 16). 

 Cartilage whose matrix is free of either form of fibril is 

 known as hyaline cartilage (fig. 159). 



Bone similarly consists of a matrix and embedded cells, 

 although here both cells and matrix undergo considerable 



