98 MANUAL OF ELEMENTARY ZOOLOGY 



are swollen into vesicles by the presence of a droplet of 

 fatty matter to which the protoplasm of the cell forms 

 a fine sheath with the nucleus on one side. In the early 

 stages of the growth of such a cell small droplets of fat are 

 laid up in the protoplasm, and these grow and run together 

 till they fill nearly the whole cell. 



Bone (Fig. 4) has a firm ground substance like cartilage, but 

 differs from the latter both in composition and in the arrange- 

 ment of its cells. The ground substance consists of an 

 organic basis impregnated with salts of lime — principally the 

 phosphate. When boiled it yields gelatin. It is arranged 

 in lamella separated by rows of minute spaces or lacuna 

 which contain the bone cells or corpuscles. The lacuna? are 



p.m. V. 



Fig. 57. — A transverse section of the spinal cord of a frog. 



a:., Central canal ; d.f., dorsal fissure ; d.h., dorsal horn ; g .m. . grey matter ; 

 n.c, large nerve cell ; p.m., pia mater ; v., vein ; z'.f ventral fissure ; 

 ?'./i., ventral horn. 



connected by fine canaliculi, through which the cells are 

 continuous by minute processes. The lamellae are arranged 

 in a concentric manner around spaces which contain blood 

 vessels. Some of these spaces are large and filled with a 

 tissue known as bone marrow, rich in blood vessels and fat 

 cells. When the layer of bone around the marrow cavities 

 is thick, it is traversed by smaller spaces, known as Haversian 

 canals, in which lie minute blood vessels. The lymph from 

 the blood vessels permeates the bone through the canaliculi. 



