94 BONE. 



nervous filaments as entering with the arteriss of the bone to the 

 sponoy and compact tissue. As far, however, 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. 



Some hold that the same is triie of the marrow, or rather the medullary mem- 

 brane ; others, among whom are Duverney and Bichat, affirm, on the contrary, 

 that the meduUarj- tissue is sensible. They state that, on sawing through the 

 bone of a living animal, and irritating the medullary membrane by i^assing a 

 probe up the cavity, or by injecting an acrid fluid, very unequivocal signs of pain 

 will be manifested. Beclard. who affirms the same fact, points out a circum- 

 stance which may account for the result occasionally turning out differently,^ 

 iiamelj\ that when the bone happens to be sa^\Ti through above the entrance of 

 the medullary artery, the nerves going along with that A'essel are divided, and 

 the marrow consequently rendered insensible, as haj^pens with any other sensible 

 part when its nerves are cut. 



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 vertebra3 and base of the skull, which afterwards 

 form the great median column to which the other parts of the bony 

 fixbric are appended. But it is by their outward form and situation 

 only, that the parts representing the future bones are then to be recog- 

 nised ; for at that early period they do not differ materially in substance 

 from the other structures of the embryo, being, like these, made up of 

 granular corpuscles or elementary cells, united together by a soft amor- 

 phous matter. 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 bone is at length 

 completed. 



But, while ifc 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 a 

 membranous tissue quite different in its nature from cartilage ; * 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 ossifica- 

 tion, which for the sake of brevity may be called the intramem'branous 

 and the intracnrtilaninous. 



Ossification in memlirane. — The tabular bones of the cranium, as 

 already said, attbrd 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 upper and greater part 

 of the frontal, and a certain portion of the occipital bones, we find 

 (except where they happen to be commencing muscular fibres) only the 



* This fact was pointed out and insisted on by Nesbitt, who distinguishes the two 

 different modes of ossification, and so far his views are quite correct. — See his Human 

 Osteogeny. Lend. 1736. 



