ANATOM Y. 
divided perpendicularly, the fibres appear oblique and de¬ 
collating each. Qther. The outer part of the invertebral 
Itgajnents is the moll folid and hard, and they gradually 
become foftcr till they are aimed in the form of a glairy 
liquor in the centre; and therefore thefe fub(lances were 
not improperly called mucous ligaments by the ancients. 
The external fibrous part of each is capable of being 
greatly extended, and of being compreiTed into a very fmall 
l'pace, while the middle fluid part is incomprellible, or 
nearly fo; and the parts of this ligament between the cir¬ 
cumference and centre approach in their properties to each 
other, in proportion to their more folid or more fluid tex¬ 
ture. The middle point is therefore a fulcrum or pivot, on 
which the motion of a ball and locket may be made, w'ith 
fuch a gradual yielding of the ftibltance of the ligament, 
in whatever direction our fpines are moved, as faves the 
body from violent (liocks, and their dangerous confequences. 
From each fide of the body of each vertebra, a bony 
bridge is produced backwards, and to one lide: from the 
poftcrior end of which one 11 anting procefs rifes, and ano¬ 
ther defeends; the fmooth, and what is generally the flat¬ 
ted, fide of each of thefe four precedes, which are called 
the oblique, is covered with a fmooth cartilage; and the 
two lower ones of each vertebra are fitted to, and articu¬ 
lated with, tire two upper or afeending oblique precedes 
of the vertebra below, having their articular ligaments 
fixed into the rough line round their edges. From between 
the oblique precedes of each lide, the vertebra is dretched 
out laterally into a procefs that is named tranfverfe. From 
the back part of the roots of the two oblique and of the 
tranfverfe procefs of each fide, a broad oblique bony plate 
is extended backwards; where thefe meet, the feventh 
procefs of the vertebrae takes its rife, and dands out back¬ 
wards : this being generally fliarp-pointed and narrow- 
edged, is therefore called fpinal procefs ; from which this 
whole chain of bones lias its name. The fubdanceof the 
precedes is confiderably Itronger and firmer, and has a 
thicker external plate, than the bodies of the vertebras 
themfelves. The feven procejfes form a concavity at their 
fore-part, which, joined to the one at the back-part of the 
bodies, makes a great hole; and the holes ol all the ver¬ 
tebra: form a long large conduit, for continuing the Ipinal 
marrow. In the upper and lower edge ot each lateral 
bridge, there is a notch. Thefe are fo adapted to each 
other in the contiguous vertebras, as to form a round hole 
in each fide between each two vertebras, through which 
the nerves that proceed from the ipinal marrow and its 
blood-veffels pafs. 
The articulations , then, of thefe true vertebrae are plain¬ 
ly double: for their bodies are joined by the intervening 
Cartilage above deferibed; and their oblique precedes, be¬ 
ing tipped with cartilages, are fo connected by theirliga- 
ments as to allow a fmall degree of motion cn every fide. 
Hence it is evident, that their centre of motion is altered 
in different politions of the trunk : for, when we bow for¬ 
wards, the upper moved part bears entirely on the bodies 
of the vertebrae ; if we bend back, the oblique precedes 
fupport the weight; if we recline to one lide, we reft upon 
the oblique precedes of that fide and part of the bodies; if 
we dand erect, all the bodies and oblique precedes have 
their (hare in our fupport. 
Hence it follows, i.That, becaufe the joints of which 
the fpine is compofed are fo numerous, the fpinal marrow, 
nerves, blood-vellels, &c. are not liable to Inch comprel- 
fion and over-dvetching in the motion of the trunk of the 
body as they would otherwife be, fince feveral vertebrae 
mult be concerned in every motion of the fpine ; and there¬ 
fore a very fmall curvature is made? at the conjunction of 
any two vertebrae. 2. That an erebl podure is the fured 
and firmed, becaufe the furface of contabl of the fulcra 
is larged, and the weight is mod perpendicular to them. 
3. That the inufcles which move the fpine act with great¬ 
er force in bringing the trunk into an erebl podure, than 
in drawing it to any other ) for, in bending forwards, back- 
Vot. I No. 35. 
545 
wards, or to a lide, the mufclbs which perform any of thefe 
actions are nearer the centre of motion"; .conlequently the 
lever with which they act is (hot ter than when the centre 
of motion is on the part of the vertebra oppolite to that 
where thefe mufcles are inferred; which is the cafe in rai¬ 
ding the trunk. This is extremely neceffary; fince, in the 
deflebtions of the fpine from a perpendicular bearing, the 
weight of the body loon inclines it in the direction we 
choofe; whereas, in railing us erebl, this great weight mud 
be more than counteracted. 4. In calculating the force 
exerted by the mufcles which move the fpine, we Ihould 
always make allowance for the abtion of the cartilages be¬ 
tween the vertebra:, which, in every motion from an erebl 
podure, mud be dretched on one fide, and compreded on 
the other, to both which they refid; whereas, in railing 
the-trunk, thefe cartilages a (lilt by their fpringy force. 
5. We are hence naturally led into the reafon of our height 
of dature increaling in the morning, and diminilhing at 
night: for the intermediate cartilages of the vertebrae, 
being preffed all day long by the weight of our body, be¬ 
come more compabl and thin in the evening; but, when 
they are relieved from this preffure in the night, they again 
expand themfelves to their former thicknefs : and, feeing 
the bulk of any part mud vary according to the different 
didenfion or repletion of the vedels compoling it, we may" 
underhand how we become taller after a plentiful meal, 
and decreafe after fading or evacuations. 6. From the 
different articulations of the bodies and oblique precedes 
of the vertebrae, and the different llrength of the liga¬ 
ments, it is plain that they are formed lo as to allow much 
larger motion forwards than backwards ; this lall being of- 
much lefs ufe, and might be dangerous, by over-llretch- 
ing the large blood-vellels that are contiguous to thebodies 
of the vertebrae. 7. The intervertebral cartilages dirivel- 
ling as they become more folid by age, is the caufe why 
old people generally bow forwards, and cannot raife their 
bodies to fuch an erebl podure as they had in their youth. 
From this general mechanifm of the fpine, an account 
is eafily deduced of all the different preternatural curva¬ 
tures of which the fpine is capable. For, if one or more 
vertebrae, or their cartilages, are of unequal thicknefs in 
oppolite fides, the fpine mull be reclined over to the thin¬ 
ner fide; which now', fultaining the greatell lhare of the 
weight, mud dill be more compreded, confequently hin¬ 
dered from extending itfelf in proportion to the other tide, 
which, being too much freed of its burden, has liberty to 
enjoy a luxuriant growth. The caufes, on which fuch an 
inequality of thicknefs in different fides of the vertebras 
depends, may vary. For either it may be owing to an 
over-didenlion of the vedels of one fide, and from thence 
a preternatural increafe of the thicknefs of that part: or, 
which is more commonly the cafe, it may proceed froman 
obllrubtion of the vedels, by which the application of 
proper nourilhment to the bony lubdance is hindered; 
whether that obllrubtion depends on the faulty difpofition 
of the vedels or fluids, or if it is produced by an unequal 
mechanical preffure occafioned by a paralytic weaknefs of 
the mufcles and ligaments, or by a fpafmodic over-abtion 
of the mufcles on any fide of the fpine, or by people con¬ 
tinuing long, or putting themfelves frequently, into any 
pollure declining from the erect one. In all thefe cafes one 
common effebt follows, to wit, the vertebras, or their car¬ 
tilages, or both, turn thick on that lide where the vedels 
are free, and remain thin on the other lide where thefe vef¬ 
fels are llraitened or obdrubled. When once we under- 
lland how thefe crooked fpines are produced, there is lit¬ 
tle difficulty in forming a jull prognofis; and a proper 
method of cure may be ealily contrived, which mull vary 
as to the internal medicines, according to the different cau¬ 
fes on which the difeale depends. But one general indi¬ 
cation mult be purfued by lurgeons ; which is, to counter- 
abl the bending force, by increaling the compredion on the 
convex part of the curvature, and diminilhing it on the 
concave lide. The manner of executing which in parti¬ 
es Z cular 
