OF THE BONES. 365 



each other. On the contrary, where it is thick, they are se- 

 parated by a layer of spongy substance, proportionate to the 

 thickness of the bone. In the bones of the skull, the inner 

 table, which is still denser, but thinner and more fragile than 

 the outer table, bears the name of vitreous lamina, and the 

 spongy substance, that of diploe. 



The short bones are formed of spongy substance, surround- 

 ed by a layer of compact substance. 



Lastly, the mixed bones, in the disposition of the two sub- 

 stances, participate in the nature of the kinds of bones to which 

 they belong. 



The two varieties of tissue, or the two substances of which 

 we have been speaking, are, in reality, one and the same tis- 

 sue, one and the same substance, differently disposed, rarefied 

 in one part and condensed in the other. A piece of compact 

 substance is exactly the same thing as a lamina or a filament of 

 spongy substance. A given longitudinal section of a long 

 bone, contains, to appearance, the same quantity of osseous 

 tissue as another equal longitudinal section of the same bone; 

 but in the one, the substance, or tissue, is condensed, and leaves 

 a large canal in its centre, while in the other, the tissue is ra- 

 refied, and the canal replaced by a multitude of spongy areolag. 

 These two substances can be transformed into each other. 

 The essential difference which they present is, so to speak, 

 foreign to them; it depends upon the presence and the pene- 

 tration of the medullary tissue, and upon its numerous vessels 

 in the very substance of the spongy bone, and upon its con- 

 tact on one of the faces only of the compact bone. 



582. The texture of the bones* is one of the points of 



* Malpighi. dt Ossium structurd f in op. posth. D. Gagliardi, Anatome 

 ossium novis inventis illustrata.Ttlomx, 1689. Cl. Havers, Ostcologia nova, &c. 

 Lond. 1691. Description exacte des os, comprise en Irois iraiUs> par J. J. 

 Courtial, J. L. Petit, et Lemery. Delascne, Mem. sur F organisation des os t 

 in Mem. de tAcad. Royak des Sciences. Paris, 1751. J. F. Reichel, de 

 Ossium ortu alque structurd. Lips. 1760. B. S. Albinus, de Constructions 

 ossiuniy in Jlnnot. Jicad. Lib. vii. cap. 17. Perenoti, Mem. sur la construc- 

 tion et sur I'accrossement des os. Mem. de Turin, t. ii. 1784. A. Scarpa, 

 de Penitiori osaium structurd commentarius. Lip3. 1795, and Paris, 1804. 



