THE OSSEOUS SYSTEM. 321 



stance, exactly like the large medullary cavities of the cylindrical bones. 

 This is most distinctly and satisfactorily shown by the examination of 

 the diaphyses of a sound or rachitic bone, but especially in the latter. 

 At the limit of the ossification itself, the osseous tissue for a distance of 

 about J to J of a line, is quite compact, without a trace of larger cavi- 

 ties, and is composed in part of the ossified matrix, and in part of car- 

 tilage cells, more or less advanced in their transformation into bone- 

 cells ("Mik. Anat.," tab. iii.); beyond this part, however, cavities, at 

 first small, and more internally, larger, come into view, the whole rela- 

 tions of which show most convincingly that they do not originate in any 

 development of the existing elements. They have an extremely irre- 

 gular contour, are oval, or roundish and angular, and for the most part 

 broader than the cartilage-cells, appearing to be eaten out, as it were, 

 in the substance of the bone, and involving severally the compact tissue, 

 matrix, and bone-cells. When the borders and limitary surfaces of these 

 spaces are closely regarded, it is, in many instances, easy to notice bone- 

 cells more or less removed, half projecting from, or buried in the wall, 

 and between them projections of the ossified matrix, so that no doubt 

 can any longer be entertained with respect to the origin of the cavities. 

 It must be confessed that there is as little to be stated, in this case, as 

 in that of the origin of the analogous cartilage-canals, and the further 

 development of the canaliculi of the bone-cells, with respect to the 

 mode in which this absorption takes place ; and the process is even still 

 more inexplicable, because, allowing that it really does take place, there 

 would then exist in the ossifying bone, at the same time and almost in 

 immediate contiguity, a formation of bone and a resolution of the tissue, 

 but very little less energetic. The above-described mode of formation 

 of the cancelli) nevertheless, is a morphological fact, and consequently, 

 the explanation of such a curious phenomenon becomes a problem to be 

 solved by chemistry and physiology. As in the diaphyses, so in the 

 ossification of all the other cartilages, medullary spaces are formed by 

 the resorption of the inner portions of that part which is half ossified. 

 But it must be stated, that these spaces do not present the same form, 

 direction, and size in every bone ; though with respect to this, it is un- 

 necessary to offer any special remarks, since the relations of this 

 primitive spongy substance are, in the main, the same as they are after- 

 wards. Still, it may be remarked, that in many bones, solitary spaces 

 are apparently developed immediately from cartilage-canals, seeing that 

 some at least of the latter, at the limit of ossification, communicate 

 directly with the spaces in the bone ; and, moreover, that not un fre- 

 quently, cartilaginous elements not yet wholly converted into bone-cells, 

 are drawn into the process of resolution. 



The medullary cavities, however they arise, are filled with a soft, 

 reddish substance foetal medulla. This substance at first consists of 



21 



