136 
HISTOLOGY OF ANIMALS. 
manent and the temporary, The first, or permanent 
variety is employed, as before stated, to supply the 
place of bone, in the skeleton of cartilaginous fishes ; 
or in the form of a membrane, either as tubes enclosing 
cavities, when it is called membraniform , or as a coating 
to the ends of bones, entering into the formation of 
joints, when it is termed articular. The second, or 
temporary form, includes the cartilage of the young of 
vertebrate animals, as well as all other kinds, which, in 
process of growth, are converted into bone. All these 
varieties of cartilage, except the articular, are covered 
with a strong layer of fibrous tissue termed perichon- 
drium, which being analogous to the periosteum of bone, 
serves as a support to the blood-vessels and nerves. 
The simplest form of cartilage, when examined 
microscopically, is found to resemble the cellular tissue 
of vegetables. It consists of a series of cells of a 
spherical or hexagonal shape, capable, in some cases, of 
being separated from each other, each of which as 
previously shown in Fig. 98 , possesses a nucleus. In 
this condition, it forms the Chorda dorsalis or rudi- 
mentary spinal column of the Lamprey , and of the 
tadpoles of the Toad and Frog. 
In the form of a firm membrane, it is employed in 
the construction of the framework of the ears of small 
mammalian animals, such as the Bat , Mouse , and Fat , 
In the Bat , especially the long-eared English variety, 
Plecotus auritus, the cartilage consists of a series of 
hexagonal cells, on an average* ^th of an inch in 
