ANATOM Y 
of the forefide, they are feparated by the pericardium and 
heart. A little more backward, they are parted in a tu¬ 
bular form by the cefophagus, to which they ferve as a 
covering; and, in the mod pofferior part, it'triangular 
fpace is left between the vertebrae and the two pleurae 
from above downward, which is filled chiefly by the aorta. 
The pleura is connected to the membranous portion of the 
fiernum, ribs, and mufcles ; to the diaphragm, pericar¬ 
dium, thymus, and velfels ; and, in a word, to whatever 
lies near its convex fide. The pleura ferves in general for 
an inner integument to the cavity of the thorax. The 
mediaftinum cuts off all communication between the two 
cavities, and hinders one lung from prefling on the other 
when we lie on one fide. It likewife forms receptacles for 
the heart, pericardium, cefophagus, &c. 
The cefophagus is a canal partly nmfcular and partly 
membranous; which commences at the inferior part of the 
pharynx, and defeends along the neck and back-part of 
the thorax, into the abdomen. While it lies in the neck, 
it is placed between the middle and left part of the cer¬ 
vical vertebrae behind the left part of the trachea ; in the 
thorax it is iituated between the layers of the pofferior 
mediaftinum, and defeends to the fourth or fifth vertebra 
of the back, in the fame direction which it had above ; 
there it inclines a little from left to right, till it reaches 
the ninth rib, to make way for the aorta; afterwards it 
inclines from right to left, and from behind forward, to 
get through the diaphragm into the upper orifice of the 
ftomaclu It is made up of feveral coats, almoft in the 
fame manner as the ftomach, of which it is the continua¬ 
tion. The firft coat, while in the thorax, is formed only 
by the duplicature of the pofferior part of the mediafti- 
pum, and is wanting above the thorax and in the neck, 
where the outer coat of the cefophagus is only a continua¬ 
tion of the cellular fubftance belonging to the neighbour¬ 
ing parts. The fecond coat is mufcular, being made up 
of feveral ftrata of lleftiv fibres. The outermoft are moffly 
longitudinal, and their quantity is much greater than that 
on the reft of the alimentary canal ; but they are not all 
continued from one end of the cefophagus to the other. 
The third is termed the nervous coat , and is like that of the 
ftomach and inteftines. It is differently folded or plaited, 
according to its length, being much wider than the muf¬ 
cular coat; and it is furrounded by a whitifh, foft, fine, 
filamentary fubftance, like a kind of cotton, which, when 
fteeped in water, fwells and grows thicker. The fourth 
or innermoft coat refembles, in fome meafure, that of the 
inteftines ; ■ except that, inftead of the villi, it has fmall 
and very Ihort papillae. It is folded lengthwife like the 
third coat; fo that the cefophagus, when cut acrofs, re- 
prefents one tube within another. Through the pores of 
this coat a vifeid lymph is continually difeharged. 
The thymus is an oblong, foft, glandular, body, round 
on the upper part, and divided below into two or three 
great lobes, of which that towards the left fide is the 
longeft. It lies between the duplicature of the ftiperior 
and anterior portion of the mediaftinum, and the great 
velfels of the heart. Among the various opinions about 
the ufe of this fubftance, fome have thought it ferved 
only to fill a part of the thorax of the foetus in the col- 
lapled date of the lungs, becaufe its fize decreafes after 
the lungs are dilated. Its particular inward ftrudture and 
fecretions are not as yet fufficiently known, fo as to enable 
us to determine its ufes; which, however, feem to be de- 
figned more for the foetus than for adults. 
The pericardium is made up of three laminae, the middle 
and chief of which is compofed of very fine tendinous fila¬ 
ments, which are belt feen in old perfons ; they are clofely 
interwoven, and crofs each other in different directions. 
The internal lamina feems to be a continuation of the outer 
coat of the heart, auricles, and great velfels. It is per¬ 
forated by an infinite number of very fmall holes, through 
which a ferous fluid continually tranfudes, in the fame 
manner as in the peritoneum ; there being no glands for 
this purpofe, as fome have fuppofed. This fluid, being 
Vox,. I. No. 38. 
605 
gradually collected after death, makes what is called aqua 
pericardii, which is found in conliderable quantities in 
opening dead bodies while they remain frelh. 
OF THE HEART. 
The heart is a mufcular body, Iituated in the cavity of 
the thorax, on the anterior part of the diaphragm, be¬ 
tween the two laminae of the mediaftinum. It is nearly 
of a conical figure, flatted on the lides, round at the top, 
and oval at the balis. Accordingly we confider in the 
heart, the bafis ; apex ; two edges, the one right and the 
other left ; and two fides, one of which is generally flat 
and inferior, the other more convex and fuperior. The 
heart is hollow within, and divided by a feptum which 
runs between the edges into two cavities, called ventriculi ; 
one of which is thick and folid; the other thin and loft. 
The latter is termed the right ventricle, the other the left. 
The right ventricle opens into the right auricle, and into 
the trunk of the pulmonary artery; the left into the left 
auricle, and into the great trunk of the aorta. At the 
edges of thefe orifices arc feveral moveable pelliculae, 
called valves ; of which fome are turned inward towards 
the cavity of the ventricles, called triglochines, or tricujpi- 
des\ others are turned towards the great veffels, called 
Jemilunares, or figmoidales. The valvulae tricufpides of the 
left ventricle are likewife termed mitrales I 
The flefliy or mufcular fibres, of which the heart i» 
made up, are difpofed in a very lingular manner, efpe- 
cially thofe of the right or anterior ventricle ; being either 
bent into arches, or folded into angles. The fibres which 
are folded into angles are longer than thofe which are only 
bent into arches. The middle of thefe arches, and the 
angles of the folds, arc turned towards the apex of the 
heart, and the extremities of the fibres towards the bafis. 
Thefe fibres differ not only in length, but in their direc. 
tions, which are very oblique in all, but much more fo 
in the long or folded fibres than in the fliort ones, which 
are fimply bent. The tubes which crofs each other tranf- 
verfely are much more numerous than thofe which crofs 
longitudinally; which ought to be taken notice of, that 
we may rectify the falfe notions that have been enter¬ 
tained concerning the motion of the heart, namely, that it 
is performed by a contortion or twilling like that of a 
ferew, or that the heart is fhortened in the time of con¬ 
traction, and lengthened in dilatation. 
The valves at the orifices of the ventricles are of two 
kinds : one kind allows the blood to enter the heart, and 
hinders it from going out the fame way; the other kind 
allows the blood to go out of the heart, but hinders it 
from returning. The valves of the firft kind terminate 
the auriculae ; and thofe of the fecond lie in the openings 
of the great arteries. The firft are termed femilunar or 
ftgmoidal valves-, the others, triglochines, tricufpidal, or mi¬ 
tral. The femilunar valves are fix in number, three be¬ 
longing to each ventricle, fituated at the mouths of the 
great arteries ; and they may be properly enough named 
valvula arteriales. Their concave fides are turned towards 
the cavity of the arteries, and their convex fides approach 
each other. In examining them with a microfcope, we 
find flefliy fibres lying in the duplicature of the membranes 
of which they are compofed. The great artery that goes 
out from the left ventricle, is termed aorta. As it goes 
out, it turns a little towards the right, and then bends ob¬ 
liquely backward, to form what is called aorta defeendens. 
The trunk of the artery which goes out from the right 
ventricle is called artcria pulmonaris. This trunk, as it is 
naturally fituated in the thorax, runs firft of all direflly 
upward for a fmall fpace; then divides laterally into two 
principal branches, which are attached to the lungs. 
The auricles are mufcular bags fituated at the bafis of 
the heart, and their capacities are in proportion to thofe 
of their refpeCtive ventricles ; one towards the right ven¬ 
tricle, the other towards the left, and joined together by 
an inner feptum and external communicating fibres, much 
in the fame manner with the ventricles; one of them being 
7 P named 
