THE OSSEOUS SYSTEM. 269 



in the direction of the tangents and radius (Fig. 110), so that the bones 

 appear to consist entirely of short thick lamellae, each of which, upon 

 closer examination, is seen to belong to two canals, and exhibits a pale 

 central line, indicating the division between the two constituent portions 

 of which it is formed. 



In the flat bones, the greater number of the canals do not run in the 

 direction of the thickness of the bone, but almost all, parallel with its 

 surface, and indeed in lines which may be conceived as radiating from 

 one point (tuber pariet ale, front ale, upper and anterior angle of the 

 scapula, articular portion of the ilium) in a penicillar or stellate manner 

 towards one or several sides ; or less frequently, as in the sternum, are 

 all parallel to each other. In the short bones, lastly, there is most 

 usually one predominant direction in which the canals run, as the ver- 

 tical in the vertebrae, that of the long axis of the extremity in the 

 carpal and tarsal bones, &c. ; it must be remarked, however, that the 

 larger processes of these bones, as, for instance, the spinous processes 

 of the vertebrae, differ in this respect from the rest of the bone, and, like 

 those of other bones, such as the coracoid and styloid processes, exhibit 

 the same disposition of the canals as that which exists in one of the 

 shorter cylindrical bones. The lamellae, fibres, and bars of the spongy 

 substance, occasionally present a few vascular canals, but only when 

 they are of some thickness. 



As the Haversian canals are vascular channels, they open in certain 

 situations : 1, externally, on the outer surface of the bone ; and, 2, 

 internally, on the walls of the medullary cavities and spaces. In both 

 situations, excessively fine and coarser pores may be everywhere per- 

 ceived, partly visible to' the naked eye, and which are more numerous 

 in proportion to the thickness of the cortex of the bone. But the rela- 

 tion of the vascular canals in the compact substance to these canals 

 thus proceeding from within and without, only partially resembles that 

 between the branches and trunks of vessels, and only in the outermost 

 and innermost lamellae of the cortical substance. In the interior of the 

 cortical portion of a bone the canals are independent, and morphologi- 

 cally may be most aptly compared to a capillary network, which* at its 

 borders is in connection at many points with larger canals. Where the 

 cortical substance rests upon the spongy substance, as in the interior of 

 the ends of the diaphyses, and in the lateral periphery of the apophyses, 

 the vascular canals are continuous, sometimes abruptly, sometimes quite 

 gradually, expanding in an infundibuliform manner, and frequently 

 anastomosing, with smaller or larger medullary spaces, so that, very 

 often, no definite limit is perceptible between them. I have never yet 

 noticed caecal terminations of the vascular canals ; it is, however, certain 

 that in many situations on the surface they must constitute, over exten- 

 sive spaces, closed networks, especially where very few or no vessels 



