ANATOMY. 
534 
definition, they have not been of the fame mind concern¬ 
ing the Species of it: for feveral think the. fyffarcofis and 
iyndefmoiis applicable to fo many joints which are univer- 
lally allowed to be clafled under, the diarthrofis, that it 
.muft create confufion to name them by any i'pecies of the 
fymphyfis. Few keep to inch a general definition of the 
iynchondroTis; and whether they determine it to allow no 
motion, or an obfeure or a manifeft one, they bring them- 
ielves into difficulties, becaufe there are examples of all 
tkefe three kinds. Some again, by too nicely diStinguifh- 
ing obfeure and manifeft motions of bones, have blended 
the fynarthrofis and diarthrofis ; and from thence have 
branched out the different compound fpecies of articula¬ 
tions that may be formed of them, fo far, that they could 
find no examples in the body to illuftrate them by. It 
would be tedious to enumerate the jarring opinions, and 
ufelefs to give a detail of the arguments adopted by the 
diiputants on this occafion. 
Before we proceed to the divifion and particular deferip- 
tion of the Skeleton, it is to be obferved, that, when the 
bones are put into their natural filiation, fcarcely any one 
of them is placed in a perpendicular bearing to another; 
though the fabric compoSed of them is lb contrived, that, 
in an ere£t pofture, a perpendicular line, for their com¬ 
mon centre of gravity, falls in the middle of their com¬ 
mon bale. On this account, vve can fupport ourfeKvcs as 
firm as if the axis of all the bones had been a ftraight line 
perpendicular to the horizon; and we have much greater 
quicknefs, eafe, and Strength, in feveral of the molt neceff 
fary motions we perform. It is true, indeed, that wherever 
the bones, on which any part of our body is fuftained, de¬ 
cline from a ftraight line, the force required in the mufcles 
to counteract the gravity of that part is greater than was 
otherwife neceffary : but then this is effectually provided 
for in fuch places, by the number and ftrength of the 
mufcles. So long, therefore, as we remain in the fame 
pofture, a confiderable number of mufcles mult be in a 
constant ftate of contraction; which we know, both from 
reafon and experience, mult foon create an uneaJy fenla- 
tion. This we call being weary of one pofture : an inconve¬ 
nience that we Ihould not have had in Handing ereCt, if the 
bearing of all the bones to each other had been perpendi¬ 
cular;" but it is more than compenfated by the advantages 
above-mentioned. The human (keleton is generally divi¬ 
ded into the Head, the Trunk, the Superior, and the Infe¬ 
rior, Extremities. 
BONES of the HEAD. 
By the head is meant all that fpheroidal part which is 
placed above the fir ft bone of the neck. It therefore com¬ 
prehends the cranium and bones of the face. 
The cranium, helmet, or brain-cafe, confifts of feveral 
pieces, w hich form a vaulted cavity, for lodging and de¬ 
fending the brain and cerebellum, with their membranes, 
veffels, and nerves. The cavity of the cranium is pro¬ 
portioned to its contents. I-lence fuch a variety of its fize 
is obferved in different Subjects ; and hence it is neither 
fo broad nor fo deep at its fore-part, in which the anterior 
lobes of the brain are lodged, as it is behind, where the 
large poflerior lobes of the brain, and the whole cerebel¬ 
lum, are contained. The roundilh figure of the Ikull, 
which makes it more capacious, and better able to defend 
its contents from external injuries, is chiefly owing to the 
equal preffure of thefe contained parts as they grow and 
increafe before it is entirely offified. It is to be obferved, 
however, that the lides of the cranium are depreffed below 
a Spherical Surface by the ftrong temporal mufcles, whofe 
action hinders here the uniform protrufion of the bones, 
which is more equally performed in other parts where no 
fuch large mufcles are. In children whole mufcles have 
not a£ted much, and confequently have not had great ef¬ 
fects on the bones, this depreflion is not fo reinarkable ; 
and therefore their heads are much rounder than in adults. 
Thefe natural caufes, differently dilpofed in different peo¬ 
ple, produce a great variety in the Shapes of Skulls, which 
is dill increafed by the different management of the heads’ 
of children when very young: fo that one may know a 
Turk’s Ikull by its globular figure, a German’s by its 
breadth and the flatnefs of the occiput, Dutch and Englilh 
by their oblong Shapes, &c. Two advantages are reaped 
from this flatnefs of the lides of the cranium, viz. the en¬ 
largement of our fphere of viiion, and more advantageous 
Situation of our ears for receiving a greater quantity of 
found, and for being lefs expofed to injuries. 
The external Surface of the upper part of the cranium, 
is very fmooth and equal, being only covered with the 
periofteunt (common to all the bones, but in the Skull’ 
diftingtiiShed by the name of pericranium,) the thin frontal 
and occipital mufcles, their tendinous aponeuroSis, and 
with the common teguments of the body; while the ex¬ 
ternal furface of its lower part has its numerous filings,/ 
deprefftons, and holes, which afford convenient origin and 
insertion to the muScles that are connected to it, and al¬ 
low Safe paSfage for the vefl'els and nerves that run through 
and near it. 
The bones of the cranium are compofed of two tables,, 
and intermediate cancelli, commonly called their diploc. 
The external table is thickeft ; the inner, from its thinnefs 
and confequent brittlenefs, has got the name of vitrea - 
Whence vve may fee the feafon of thofe mifehievous con- 
fequences which fo often attend a collection of matter in 
the diploe, either from an internal or external caufe, be¬ 
fore any fign of fuch a collection appears in the teguments, 
which cover that part of the Ikull where it is lodged. 
The cranium confifts of eight bones, Six of which are 
faid to be proper, and the other two are reckoned com¬ 
mon to it and to the face. The fix proper are the os 
frontis, two offa parietalia, two ojja temporum, and the os 
occipitis. Tire common arc the os ethmoides and fphenoides . 
The os frontis forms the whole fore-part of the vault 
the two offa parietalia form the upper and middle part of 
it; the offa temporum compofe the lower part of the lides; 
the os occipitis makes the whole hinder part, and fome of 
thebafe; the os ethmoides is placed in the fore-part of 
the bale, and the os fphenoides in the middle of it. 
Thefe bones are joined to each other by five futures j 
the names of which are the coronal, lambdoidal, fagillaf 
and two fquamous. The coronal future is extended over 
the head, from within about an inch of the external can- 
thus of one eye, to the like distance from the other; which 
being near the place where the ancients wore their vitta?, 
coronae, or garlands, has thence got its name. 
The lambdoidal future begins farther back than the 
vertex or crown of the head, whence its two legs are 
Stretched obliquely downwards, and are now generally faid 
to extend themfelves to the bale of the Skull: but for¬ 
merly anatomifts reckoned the proper lambdoidal future 
to terminate at the fquamous futures; and what is ex¬ 
tended at an angle down from that on each fide, where 
the indentations are lefs confpicuous than in the upper 
part of the future, they called aditamentum futurce lambdoi- 
dis. This future is fometimes very irregular, being made 
up of a great many fmall futures, which furround fo ma¬ 
ny little bones that are generally larger and more confpi¬ 
cuous on the external furface of the Ikull than internally. 
Thefe bones are commonly called triquetra or Wormiana: 
but fome other name ought to be given them, for they are 
not always of a triangular figure, and older anatomifts 
than Wormius have deferibed them. 
The fagittal future is placed longitudinally in the mid¬ 
dle of the upper part of the Skull, and commonly termi¬ 
nates at the middle of the coronal and of the lambdoidal 
futures; between which it is faid to be placed, as an ar¬ 
row is between the Siring and the bow. However, this 
future is frequently continued through the middle of the 
os frontis down to the root of the nofe; which, fome fay, 
happens oftener in women than in men. 
The fquamous agglutinations, or falfe futures, are one on 
each fide, a little above the ear, of a Semicircular figure, 
formed by the over-lapping (like one fcaie upon another) 
of 
