8 DEVELOPMENT OP BOXE. 



constitute the permanent areolae and Haversian canals. With a high 

 magnifying power, the delicate ossific fibres here alluded to are seen 

 themselves to be composed of minute cells having an elliptical form 

 and central nuclei. These cells attract into their interior the cal- 

 cerous salts of the blood, and their nuclei become developed, as I 

 believe, into the future corpuscles of Purkinje. It is possible also 

 that some of the cartilage cells become corpuscles of Purkinje in the 

 fully developed bone. 



During the progress of the phenomena above described, the contents 

 of the cells undergo certain changes. At first, their contents are 

 transparent, then they become granular, and still later opaque, from 

 the presence of amorphous matter mingled with nuclei, nucleoli, and 

 the remains of secondary cells. In the latter state they also contain 

 an abundance of minute oil-globules. These latter increase in size as 

 the ossific changes advance, and in the newly formed osseous areola? 

 they are very numerous and have attained the ordinary size of adipose 

 cells. 



Cartilaginification is complete in the human embryo at about the 

 sixth week ; and the first point of ossification is observed in the 

 clavicle at about the seventh week. Ossification commences at the 

 centre, and thence proceeds towards the surface ; in flat bones the osse- 

 ous tissue radiates between two membranes from a central point to- 

 wards the periphery, in short bones from a centre towards the circum- 

 ference, and in long bones from a central portion, diaphysis, towards a 

 secondary centre, epiphysis, situated at each extremity. Large pro- 

 cesses, as the trochanters, are provided with a distinct centre of 

 development, which is named apophyais. 



The groicth of bone in length takes place at the extremity of the 

 diaphysis, and in bulk by fresh deposition on the surface ; while the 

 medullary canal is formed and increased by absorption from within. 



The period of ossification is different in different bones ; the order 

 of succession may be thus arranged : 



During the fifth week, ossification commences in the clavicle, lower 

 jaw, and upper jaw. 



During the sixth week, in the femur, humerus, tibia, radius, and 

 ulna. 



During the seventh and eighth weeks, in the fibula, frontal, occi- 

 pital, sphenoid, ribs, parietal, temporal, nasal, vomer, palate, vertebra?, 

 three first pieces of sacrum, malar, metacarpus, metatarsus, third 

 phalanges of the hands and feet, and ilium. 



During the third month, in the first and second phalanges of the 

 hands and feet, lachrymal bone, and ischium. 



During the fifth month, in the mastoid portion of the temporal, 

 ethmoid, inferior turbinated, sternum, os pubis, and two last pieces of 

 sacrum. 



During the sixth month, in the body and odontoid process of the 

 axis, and calcaneus. 



