268 BONE OR OSSEOUS TISSUE. 



vessels (fig. 297, T), and often partially or wholly enclosing them (perivascular). The 

 lymph or plasma of the blood is enabled to penetrate the hard bony substance by 

 means of the lacunas and communicating canaliculi, which appear to bear the same 

 relation to the lymphatics as do the cell-spaces of ordinary connective tissue to the 

 lymphatics of that tissue. 



The fine nerves which may be seen entering the bones along with the arteries 

 are probably chiefly destined for those vessels ; it is not known whether any end in 

 the bony tissue itself. 



As far as can be judged from observations on man and experiments on the lower 

 animals, the bones, as well as their investing periosteum, are scarcely if at all sensible 

 in the healthy condition, although they are painfully so when inflamed. 



FORMATION AND GROWTH OF BONE. 



The foundation of the skeleton is laid at a very early period ; for, among the 

 parts that appear soonest in the embryo, we distinguish the rudiments of the vertebrae 

 and base of the skull, which afterwards form the great median column to which the 

 other parts of the bony fabric are appended. But it is by their outward form and 

 situation only, that the parts representing the future bones are then to be recognised ; 

 for at that early period they do not differ materially in substance from the other 

 structures of the embryo, being made up of mesoblastic cells, with a small amount 

 of intercellular substance. Very soon, however, they become cartilaginous, and 

 ossification in due time beginning in the cartilage and continuing to spread from 

 one or from several points, the bony tissue becomes gradually formed. 



But, while it is true with respect to the bones generally that their ossification 

 commences in cartilage, it is not so in every instance. The tabular bones forming 

 the roof of the skull may be adduced as a decided example to the contrary ; in these 

 the ossification goes on in connective tissue altogether unconnected with any carti- 

 lage ; and even in the long bones, in which ossification undoubtedly commences and 

 to a certain extent proceeds in cartilage, it will be afterwards shown that there is 

 much less of the increment of the bone really owing to that mode of ossification than 

 was at one time generally believed. It is necessary, therefore, to distinguish two 

 species or modes of ossification, which for the sake of brevity may be called the 

 intramembranous and the intracartilaginous, 



INTBAMEMBBANOTJS OSSIFICATION: OSSIFICATION IN CONNECTIVE TISSUE. 



The tabular bones of the cranium, as already said, afford an example of this mode 

 of ossification. The base of the skull in the embryo is cartilaginous ; but in the 

 roof, that is to say, the part comprehending the parietal, the frontal, and a certain 

 portion of the occipital bones, we find (except where there happen to be commencing 

 muscular fibres) only the integuments, the dura mater, and an intermediate layer, in 

 which the ossification proceeds. 



The commencing ossification of the parietal bone, which may be selected as an 

 example, appears to the naked eye in the form of a network in which the little bars 

 or spicula of bone run in various directions, and meet each other at short distances. 

 By-and-by the ossified part, becoming extended, gets thicker and closer in texture, 

 especially towards the centre, and the larger bony spicula which now appear, run out 

 in radiating lines to the circumference. The ossification continues thus to spread 

 and consolidate until the parietal meets the neighbouring bones, with which it is at 

 length united by sutures. 



Fig. 308 represents the parietal bone of an embryo sheep about two inches and 

 a half long, and shows the character of the ossification as it appears when the object 



