SECT. 106.] OSSEOUS SYSTEM. 205 



depositions upon their walls. Finally, some become thus closed, 

 others are converted into true vascular canals, their contents 

 developing vessels from the primitive formative cells, which now 

 appear as marrow-cells ; these vessels then communicate with those 

 of the periosteum. When such a bone has thus far advanced in 

 its development, the subsequent changes are easily understood. 

 By means of the blastema, which is continually formed at its 

 borders and surfaces, it continues to grow horizontally and in 

 thickness, until it has attained its typical form and size, and in 

 its interior, by solution of its compact substance, spongy sub- 

 stance (or even larger cavities) is subsequently formed ; so that, 

 like a bone which has been developed from cartilage and perios- 

 teal deposition, it finally contains, externally, compact substance 

 with Haversian canals, and internally medullary spaces, but with 

 distinct secondary deposits. 



In concluding this account of the development of the bones, 

 I may annex some remarks as to the periods of time in 

 which the various stages are gone through. Valentin saw the 

 cartilaginous basis of the ribs in a human embryo six lines long. 

 From the sixth to the seventh week, that of the cranium can be 

 distinctly recognised, also that of the vertebras and of the sus- 

 pensory zones of the limbs ; that of the limbs themselves, at a 

 somewhat later period (from the eighth to the ninth week). The 

 ossification begins in the second month, first in the clavicle and 

 lower jaw (fifth to the seventh week), then in the vertebras, 

 humerus, femur, the ribs and the cartilaginous part of the flat 

 portion of the occipital. At the end of the second and com- 

 mencement of the third month, the frontal bone, the scapula, the 

 bones of the fore-arm and leg and upper jaw make their appear- 

 ance. In the third month, the remaining cranial bones, with but 

 few exceptions, begin to ossify, also the metatarsus and metacarpus 

 and the phalanges ; in the fourth month, the iliac bones and ossicles 

 of the ear; in the fourth or fifth, the ethmoid, conchas, sternum, 

 pubis and ischium ; from the sixth to the seventh month, the 

 calcancum and astragalus ; in the eighth month, the hyoid bone. 

 At birth, the epiphyses of all cylindrical bones, occasionally with 

 the exception of those of the femur and tibia which are directed 

 towards each other; further, all the bones of the carpus, the five 

 smaller ones of the tarsus, the patella, sesamoid bones, and the 

 last pieces of the coccyx are still unossified. From the time of 

 birth to the fourth year, osseous nuclei make their appearance 

 also in these parts, but in the pisiform bone not until the twelfth 



