4 DEVELOPMENT OF BONB. 



surface of a bone, and largest near its centre, where they gradually 

 merge into cancelli ; by the frequent communications of their branches 

 they form a coarse network in the basis substance. 



The cells of bone, or corpuscles of Purkinje, are thickly disseminated 

 through the basis substance ; they are irregular in size and form, give 

 off numerous minute branching tubuli which radiate from all parts of 

 their circumference, and in the dried state of the bone contain merely 

 the remains of membranous cells and some calcareous salts.* In the 

 living bone the cells and their tubuli are probably filled with a nutri- 

 tive fluid holding calcareous salts in solution. The form of the cells is 

 oval or round and more or less flattened, their long diameter corre- 

 sponds with the long axis of the bone, and their tubuli cross the direc- 

 tion of the lamellae and constitute a very delicate network in the basis 

 substance by communicating with each other, and with the tubuli of 

 neighbouring cells. The tubuli of the cells nearest the Haversian 

 canals terminate upon the internal surface of those cavities. The size 

 of the cells varies in extreme measurement from g6 6g to ^1^ of an 

 inch in their long diameter, an ordinary average being 10 I 6tf ; the 

 breadth of the oval cells is about one half or one third their length, and 

 their thickness one half their breadth. They are situated between the 

 lamellae, to which circumstance they owe their compressed form. 



In the fresh state, bones are invested by a dense fibrous membrane, 

 the periosteum, which covers every part of their surface with the ex- 

 ception of the articular extremities, the latter being coated by a thin 

 layer of cartilage. The periosteum of the bones of the skull is termed 

 pericranium ; and the analogous membrane of external cartilages, peri- 

 chondrium. Lining the interior of the medullary canal of long bones, 

 the Haversian canals, the cells of the cancelli, and the cells of short, flat, 

 and irregular bones, is the medullary membrane, which acts as an in- 

 ternal periosteum. It is through the medium of the vessels ramifying 

 in these membranes that the changes required by nutrition occur in 

 bones, and the secretion of medulla into their interior is effected. 

 The medullary canal, Haversian canals and cells of long bones, and 

 the cells of other bones, are filled with a yellowish oily substance, the 

 medulla, which is contained in a loose areolar tissue formed by the 

 medullary membrane. 



Development of Bone. To explain the development of bone it is ne- 

 cessary to inform the student that all organised bodies, whether belong- 

 ing to the vegetable or the animal kingdom, are developed primordially 

 from minute vesicles. These vesicles, or, as they are commonly 

 termed, cells, are composed of a thin membrane containing a fluid or 

 granular matter, and a small rounded mass, the nucleus, around which 

 the cell was originally formed. Moreover, the nucleus generally con- 

 tains one or more small round granules, the nucleolus or nucleoli. "From 

 cells having this structure all the tissues of the body are elaborated ; 



* MUller and Henle conceived that the bone cells and tubuli were the prin- 

 cipal seat of the calcareous matter. Hence they have been named culcigerous 

 cells and tubuli. 



