i6 



HISTOLOGY. 



extending gradually in all directions until the final result is 

 much the same. In structure cartilage bones and membrane 

 bones are very similar, but the differences in the history is very 

 important, as will be seen later in dealing with the skeleton. 



Closely allied to bone is the dentine of teeth and scales, the 

 chief differences lying in the greater density of the intercellular 

 substance, and in the fact that the dentine-producing cells 

 (odontoblasts) do not become included in the solid structure. 

 The same fine protoplasmic processes of the cells exist, lying in 

 dentinal canals which pursue a nearly parallel course. 



Blood and lymph are connective tissues, with a fluid intercel- 

 lular substance (plasma) in which float the cells. In lymph the 



cells are all of one type, known as leu- 

 cocytes, white in color, and possessed 

 of marked amoeboid powers. In blood, 

 besides the leucocytes (white corpus- 

 cles), there are also numerous red cor- 

 puscles, the source of color of the blood. 

 These red corpuscles have no amoeboid 

 powers, but are merely the means of 

 transferrence of oxygen and carbon di- 

 oxide 1 to and from the tissues. In 

 the lower vertebrates the red corpus- 

 cles are oval and nucleated ; in the mammals the nuclei are lost, 

 and the corpuscles are usually biconcave, circular disks. The 

 blood plaques may also be mentioned. 



1 Carbon dioxide is also carried by the plasma. 



FIG. 19. a, b, c, red blood 

 corpuscle of man; d, white 

 corpuscle of man; <?, red cor- 

 puscle of frog. 



