VOL. XXXI.] PHILOSOPHICAL TRANSACTIONS. 485 



part of the bone. And those bony particles that lie parallel to the length of the 

 bone, send out vessels from their sides, that issue out through the side of the 

 bone. It is impossible to conceive the prodigious number of small vessels, of 

 which the cortical part of the bone consists ; which in some places lies no 

 thicker on the spongy part of the bone than a thick hair of a man's head, 

 though in other places it has 3 or 4 times that thickness. 



The periosteum is united to the cortex of the bone, not only on the outside, 

 but even by entering in many places into the very substance of the bone, and is 

 joined to it by the vessels, which issue out from the bone, in such a manner, 

 that sometimes one cannot determine which is the bone, and which belongs to 

 the membrane investing it, both appearing in the microscope to consist alike of 

 exceedingly small vessels. 



Fig. 3, pi. 13, is a representation of a small part of the bone, with the 

 periosteum adhering to it ; in which abcdep represents the bony part ; bghie 

 the periosteum, the thickness of which is designed by bg, or ie; though in 

 other places of the bone, and even at no greater distance than 2 or 3 hairs 

 breadth, it is twice or thrice as thick : all the small vessels in the periosteum are 

 represented by so many dots or points ; but in other places, where I had several 

 times seen the membrane of twice- this thickness, the upper half of it has ap- 

 peared to be of a different make from the under part," for as much as in the 

 upper part I could discover not only those vessels thai had been cut transversely, 

 and which consequently were represented by so many points, but likewise a 

 great number of other vessels running lengthwise along the membrane, as is 

 represented by lopqnm, in fig. 4. 



I am fully persuaded, that the part represented by bghie, fig. 3, is not 

 entirely membranous, but that some part of it is really bony. If we cut 

 through the periosteum so deep as to divide the part of the bone marked with 

 the letters abcdep, in the same figure, we find the same appearance of pores in 

 the bony substance, which are no other than the transverse sections of small 

 vessels ; and besides these, there are other vessels running lengthwise in the 

 bone. And we find just the same in those transparent parts, that lie between 

 the bony particles, which are represented thicker between bode, than they ap- 

 peared to me. 



It is my opinion, that the use of these bony particles, is to convey an 

 oleaginous liquor into the periosteum, and from thence, by the intervention of 

 the other membranes, into all parts of the body, when in a healthful state. 



In another place, I saw a great number of vessels rising from a greater depth 

 within the bone, which drew closer together, so as to compose small fasciculi, 

 before they entered the periosteum, in which they separated from each other. 



