THE OSSEOUS SYSTEM. 335 



I would further rcmailc, that the secondary bones, so long as they are 

 in a growing state, are much more vascular than afterwards, even ex- 

 ceeding, in this respect, the periosteal layers of the other bones; on 

 which account their medulla, containing the multi-nuclear, enigmatical 

 bodies, already referred to, is of a redder color. The vessels enter these 

 bones at innumerable points on the surface, and, in the different bones, 

 run in vertical or horizontal canals. The latter is the case in the flatter 

 bones, in \Yhich the vascular channels run principally in the longitudinal 

 direction of the osseous rays proceeding from the primary point of ossi- 

 fication; and the former, in consequence of which the surface of the 

 bone frequently presents an extremely delicate, millepore-like aspect, 

 occurs in the thicker portions, A great many of these canals after- 

 wards become obliterated, or, at all events, very much contracted, whence 

 the surface of the bone is rendered smoother. 



In conclusion to these remarks on the development of the bones, I 

 will add a few words regarding their conditions at different periods. 

 Valentin noticed the cartilaginous rudiments of the ribs in a human 

 embryo 6 lines long. That of the cranium is distinctly recognizable in 

 the sixth or seventh week, as well as those of the vertebral zone and that 

 of the extremities ; those of the extremities proper do not appear till later 

 (in the eighth or ninth week). Ossification commences as early as the 

 second month, first in the clavicles and lower jaw (fifth to seventh week), 

 then in the vertehrce, the humerus, femur, ribs, and the cartilaginous por- 

 tions of the lamina of the occipital bone. At the end of the second, and 

 beginning of the third month, ossification is apparent in the frontal bone, 

 scapidce, bones of the fore-arm and leg, and upper jaw ; in the third month, 

 in the rest of the cranial bones, with few exceptions, the metacarpal and 

 metatarsal bones, and phalanges ; in the fourth month, in the ilium and 

 ossieula auditus ; in the fourth or fifth month in the ethmoid, the turbi- 

 nated bones, the sternum, pubis, and ischium; in the sixth to the seventh 

 month, in the os calcis, and astragalus; in the eighth month, in the os 

 hyoides. At birth, the epiphyses of all the cylindrical bones are still 

 unossified, with the occasional exception of those at the lower extremity 

 of i\\Q femur and upper end of the tibia ; and besides these all the carpal 

 and the five smaller tarsal bones, the patella, sesamoid bones, and the 

 last segment of the coccyx. After birth, up to the fourth year, the 

 nuclei of these bones also make their appearance ; but, in the os pisiforme, 

 not till the twelfth year. The union of most of the epiphyses and pro- 

 cesses, with the diaphyscs takes place, in part at the time of puberty, in 

 part towards the end of the period of growth.* 



* [Dr. Sharpey's discovery that certain bones of the skull are developed in the same man- 

 ner as those layers which are formed under the periosteum in the long bones, has been a 

 sort of apple of discord among histologists, and has produced a great variety of controversies 

 not only among them, but among comparative anatomists ; controversies wliose heat has 



