CARTILAGE AND BONE 



61 



for mechanical reasons, of the retention of a cartilaginous stage of 

 growth in the young is obvious. In some regions, however, true 

 bone is developed in connective tissue without being preceded by 

 cartilage. This is always the case in the exoskeleton. To such 

 ossifications the terms ' dermal ' and ' membrane ' bone have been 

 given. 



It was a London physician, Clopton Havers, who first began 

 the serious study of the structure of bone. Towards the end of the 

 seventeenth century he described those small canals which have 

 since borne his name. The Haversian canals are minute branching 

 channels running throughout the substance of bone, and containing 



ff/ 



Flo. 42. 



Transverse section of a human humerus showing the structure of bone, with lacunae for 

 bone-cells set in concentric rings round three Haversian canals. (From Quain's Anatomy by 

 permission of the Publishers.) 



blood-vessels, lymph-spaces, and nerves. To Purkinge is due the 

 discovery of the microscopic cavities in which, as Virchow after- 

 wards showed, the cells or so-called bone-corpuscles are situated. 

 Among the numerous observers who have since completed our 

 knowledge of this complex tissue may be mentioned Tomes, Todd 

 and Bowman, Sharpey, and Kolliker. 



Bone is constituted by lamellae formed of an organic basis, 

 ossein, which yields gelatine ; it is impregnated with phosphate and 

 carbonate of lime. Distributed throughout the matrix, between the 

 lamellae they have secreted, are the bone-cells (Fig. 42). These 

 are provided with a multitude of fine ramifications reaching from 

 one to the other, and ultimately to the nearest Haversian canal, or 

 to the surface of the bone. The lamellae are arranged concentrically 



