24 Of Bones . 
made up of a vaft many fmall veflels, fome running 
lengthways, others tending towards the Tides of the bony 
particles, fome lying parallel, and others perpendicular 
to the length of the bone ; thefe laft have veflels pro¬ 
ceeding from their extremities; and others that compofe 
the cortex, or fuperficial part of the bone, proceed from 
their Tides; thofe long particles, which lie parallel to the 
length of the bone, emit veflels from their Tides, that 
iflue out through the fide of the bone. It is impofiible 
to conceive what a prodigious number of fmall veflels 
compofe the cortical part of the bone, which on the 
fpongy part in fome places is no thicker than a human 
hair; though, in others, three or four times that thicknefs. 
The periofteum is united to the cortex of the bone, 
not only to the outfide of the cortex, but even by en¬ 
tering into its very fubftance in feveral places, and is 
joined thereto by the veflels which proceed from the bone. 
Fig. 51. reprefents a fmall part of the bone with the 
perioflreum adhering to it; ABCDEF ftiews the bony 
part, BGHIE the periofteum, in which all the fmall 
veflels are reprefented by dots. In other places where 
the thicknefs was twice as much, not only thofe veflels 
that had been tranfverfly cut, and confequently reprefented 
by many points; but alfo many other veflels running 
lengthways, as in fig. 52. LOP Q_N M and that part 
reprefented by B G H I E, fig. 51. are not entirely mem¬ 
braneous, but really bony. KLMNA reprefent the 
bony part, in which though no pores or veflels are here 
reprefented, yet it is full of openings. Fig. 53./R S W 
XTV reprefents a part of another bone, S WXT the 
periofteum, which in this place was no thicker than a 
large hair of a man’s beard; but in another part of the 
fame, and at a fmall diftance, it was four times that 
thicknefs* In another piece of bone To placed before the 
microfcope 
