270 PALEONTOLOGY. 



FOUKTH DIVISION, 



VERTEBKATA, Cuvier. SPINT-CEREBRATA, Grant. 



This great division comprehends all animals that have an 

 internal articulated skeleton, symmetrical in the disposition 

 of its elements, and destined to protect the brain and spinal 

 cord, lodge the organs of the special senses, and form a 

 framework for the organs of locomotion. 



Bone is composed of an animal substance, in .the -inter- 

 stices of which earthy salts, chiefly the phosphate of lime, 

 are deposited. A thin slice of bone, mounted in Canada 

 balsam, and examined by a good microscope, with a power of 

 two hundred linear, exhibits in a very satisfactory manner 

 the osseous structure. It is seen to be composed of a 

 number of openings in a delicate transparent membrane. 

 The openings are transverse sections of canals that every- 

 where traverse bone, and are called Haversian, after their 

 discoverer. 



Around each canal we observe a number of fine concentric 

 lines, each line having a laminated structure ; between the 

 Iamina3 a number of singular spider-like bodies are situated. 

 These are the bone-cells, which are in general oval, and have 

 a number of microscopic tubes radiating from their circum- 

 ference. In recent bones the cells are transparent, but in 

 fossil bones they are always opaque, and the same has been 

 observed in the bones of mummies. In the one case the 

 process of fossilisation, in the other- the percolation of bitu- 

 minous matter, has injected the cells through the small tubes 

 that communicate with the Haversian canals. 



Mr. John Quekett has shown, in a valuable paper on the 

 intimate structure of bone, that the bone-cells of each class 

 of the Vertebrata have forms and dimensions that are special 

 to each. The bone-cells of the Mammalia average about 

 fftny "o of an i ucn i 11 the long diameter ; and if we take this as 

 a standard of comparison, we find that the bone-cells of birds 

 will fall below it, and the bone-cells of reptiles will far exceed 

 it ; whilst those of fishes are so entirely different from mam- 

 mals, birds, and reptiles, both in shape and size, that they 



