THE INTRODUCTION 
and fometimcs in difeafed or wounded limbs, (Tab. 1.) they decreafe as 
well as the flefliy parts, though not fo faft becaufe of their hardnefs. 
Sometimes the oflifying matter flows out of the bones, and forms bony 
excrcfccncies : (Tab. xlii. xlvi. and li.) In old men it fometiines fixes on 
the arteries, and makes them grow bony; and when this happens to a de¬ 
gree, the arteries lofe their power to propel the blood, and then the ex¬ 
treme parts mortify: Sometimes membranes and other parts oflify. The 
moft extraordinary cafe of this kind I have ever known, was of part of the 
mufcular fibres of the heart. (Tab. i.) I have alfo known one inftance of 
a deficiency of this oflifying matter, in the lower jaw of an adult body; 
(Tab. ix.) where all that part on one fide, which is beyond the teeth, was 
of a fubftance between that of a cartilage and a ligament. In children that 
have died of the rickets, I have found the nodes on the bones foft, Ipongy 
and bloody, and in one fubjedl feveral of them as limber as leather, and 
the periofteum infome places many times its natural thicknels; but the 
cartilages and cartilaginous epiphyfes had no apparent alteration in their 
texture, though fome were fwelled to more than twice their natural 
diameters. 
Every cylindrical bone has a large middle cavity, which contains an oily 
marrow, and a great number of lefler cells towards their extremities, 
which contain a bloody marrow. (Tab. i. and ii.) The bloody marrow is 
alfo found in all fpongy cells of bones. (Tab. xiii.) The ufe of the firfl 
kind of marrow I imagine is t-n fnft-en, and render lefs brittle, the harder 
fibres of bones near which it is feated; and that the other marrow is of 
the fame ufe to the lefs compact fibres, which the more oily marrow 
might have made too foft; and that for this realon, there is lefs of the oily 
marrow, and more of the bloody in young bones than in old ones. Every 
one of thefe cells is lined with a fine membrane, and the marrow in the 
larger cells is alfo contained in thin membranous vdicles; in which mem¬ 
branes the veflels are fpread, which enter in obliquely, about the middle 
of the cylindrical bones, from fome of whofe branches the marrow is fe- 
creted, while others of them enter the internal fubftance of the bones for 
their nourishment; and the realon why they enter obliquely is, that they 
may not weaken the bones by dividing too many fibres in the fame place. 
If the bones had been formed of the fame quantity of matter without 
any cavities, they would if they were ftreight be able to fuftain the fame 
weight: But being made hollow, their ftrength to refill breaking tranf- 
verlly is encreafed as much as their diameters are encreafed, without en- 
creafing their weights, which mechanifm being yet more convenient tor 
