: above downward, it inclines towards the left 
| side; and that if before the thorax is opened, 
| a sharp instrument be run through the middle 
of the sternum, therevrill be almost the breadth 
of a finger between 'the instrument and the 
! mediastinum, provided that the sternum re- 
main in its natural situation, and the cartilages 
of the ribs be cut at the distance of an inch 
from it on each side. 
From all this, we see not only that the 
; thorax is divided into two cavities, entirely 
separated from each other by a middle septum, 
Without any communication, but also that by 
; the obliquity of this partition, fhe right cavity 
i is greater than the left ; but thefts are cxcep- 
[ tions to the above descriptions. Lientand 
| says he has met with several subjects in which 
the mediastinum descended along the middle 
• of /the sternum ; and ethers, where it was in- 
| dined to tiie left side. Sabatier observes this 
i is rare; but he has likewise met with several 
examples, where an instrument thrust through 
the middle of the sternum, got into the left 
cavity of the thorax. And he has sometimes 
seen the right lamina of the mediastinum fixed 
to the middle of the sternum, while the left 
; one was fixed opposite to the articulation, 
i with the cartilages of the ribs ; a space being 
left between the two, which was Idled with 
cellular substance intermixed with fat. 
The pleura is connected to the membra- 
nous portion of the sternum,’ ribs, and mus- 
cles ; to the diaphragm, pericardium, thymus 
gland, and vessels : and', in a word, to what- 
ever lies n< ar its convex side. 
The surface of the pleura turned to the ca- 
vity of the breast, is continually moistened by 
a lymphatic serositV, which transudes through 
the pores' of the membranous portion. This 
fluid is said to be secreted by imperceptible 
glands; but the existence of these glands has 
not been hitherto demonstrated. 
Use. The pleura serves in general for an 
inner integument to the cavity of the thorax. 
The mediastinum cuts oil’ all communication 
between the two cavities, and hinders one 
lung from pressing on the other when we lie on 
one side. It likewise forms receptacles for 
the heart, pericardium, oesophagus, &c. and 
it is continued over the lungs in the manner 
which shall be explained hereafter. 
Before we leave the pleura it must be ob- 
served, that it adheres firmly to the ribs. This 
adhesion keeps the pleura stretched, and hin- 
ders it from slipping, or giving way. It like- 
wise renders this membrane extremely sensible 
of the least separation, caused by a coagulable 
lymph or accumulated blood ; the nervous 
filaments being likewise in this case very much 
compressed in inspiration by the swelling of 
the intercostal muscles. 
Pericardium. 
The heart, with all the parts belonging to it, 
is contained in a membranous capsula, called 
pericardium, which is, in some measure, of a 
conical figure, and somewhat bigger than the 
heart; but the difference must be less during 
life, when tire heart is full of blood. It is not 
fixed to the basis of the heart, but round the 
large veins above the auricles, before they send 
off the ramifications, and round the large arte- 
ries before their division. 
The pericardium is made up of three lami- 
na- ; the middle and chief of which is composed 
of very fine tendinous filaments, which are 
best seen in old persons ; they are closely in- 
ANATOMYr 
terwoven, and cross each other in different di- 
rections, The internal lamina seems to be a 
continuation of the outer coat of the heart, 
auricles, and great vessels. The trunks of the 
aorta and pulmonary artery 1 have one com- 
mon coat, which contains them both, as in 
a sheath, and is lined on the inside bv a cel- 
lular substance, chiefly in that space which 
lies between where the trunks are turned to 
each other and the sides of the sheath. There 
is but a very small portion of the vena cava 
contained in the pericardium. 
It is the middle lamina which chiefly forms 
the pericardium; and the figure of this bag is 
not simply conical, its apex or point being 
very round, and the basis having a particular 
elongation, which surrounds the great vessels, 
as lias been already said, as amply as the other 
portion surrounds the heart. 
The pericardium is closely connected to 
the diaphragm, not at the apex, but exactly 
at that place which answers to the flat or lower 
side of the he;irt ; and it is a very difficult 
matter to separate it from the diaphragm in 
dissection, the tendinous fibres of the one sub- 
stance intermixing, with those of the other. 
This adhering, portion is in some measure of a 
triangular shape, answering to that of the 
lower side of the heart; and the rest of the 
bag lies upon the diaphragm, without any ad- 
hesion. 
The external lamina, or common covering, 
as it may be, called more properly, is formed 
by the duplicature of the mediastinum. It 
adheres to the proper bag of the pericar- 
dium by the intervention of the cellular sub- 
stance in that duplicature, but leaves it where 
the pericardium adheres to the diaphragm ; 
on the upper surface of which it is spread, 
as being a continuation of the pleura. 
'The internal lamina is perforated by an 
infinite number of very small holes through 
which a serous fluid continually transudes, 
in the same manner as in the peritonaeum, 
there being no glands for this purpose as 
some have supposed. This fluid being ge- 
nerally collected after death, makes what is 
called aqua pericardii, which is found in con- 
siderable quantities in opening dead bodies 
while they remain fresh. Sometimes it is of 
a reddish colour, which may be owing to a 
transudation of blood through the fine mem- 
brane of the auricles. 
Of the Heart. 
Situation in general, and conformation. 
The heart is a muscular body situated in the 
cavity of the thorax on the anterior part of 
the diaphragm between the two lamina; of 
the mediastinum. It is in some measure of 
a conical figure flatted on fhe sides, round 
at the top, and oval at the basis. According- 
ly we consider in the heart, the basis ; the 
apex ; two edges, the one right and the other 
left ; and two sides, one of which is generally 
flat and inferior, the other more convex and 
superior. 
Besides the muscular body, which forms 
what we chiefly call the heart, its basis is 
accompanied by two appendices called auri- 
cula;, and by large blood-vessels; of which 
hereafter : and all these are included in the 
pericardium. 
The heart is hollow within, and divided by 
a septum which runs besween the edges into 
two cavities called ventriculae, one of which 
67 
is thick and solid, the other thin and soft. 
This latter is generally termed the right ven- 
tricle, the other the left ventricle; though 
in their natural situation, the right ventricle 
is placed more anteriorly than the left, as we 
shall see. hereafter. 
■Each ventricle opens to the basis by two 
orifices ; one of which answers to the auricl. s, 
the other to the mouth of a large artery ; and 
accordingly, one of them may be termed 
the auricular orifice, the other the arterial 
orifice. 1’he right ventricle opens into the 
right auricle, and into the trunk of the pul- 
monary artery ; the left into the left auricle, 
and into the great trunk of the aorta. At the 
edges of these orifices are found several 
moveable pellicula;, called valves, of which 
some are turned inward toward the cavity of 
the ventricles called the tricuspid ; others 
are turned toward the great vessels, called 
semilunar, or sigmoid, 'fhe tricuspid valves 
of the left ventricle are likewise termed mi- 
tral. 
Ventricles . The inner surface of the ven- 
tricles is very uneven, many eminences and' 
cavities being observable therein. The most 
considerable eminences are thick fleshy pro- 
ductions, called columnae carniaq fleshy co- 
lumns. To the extremities of these pillars 
are fastened several chorda; tending, tendi- 
nous cords, the other ends of which are join- 
ed to the valvulae tricuspides. There are: 
likewise other small short tendinous ropes 
along both edges of the septum, between 
the ventricles. These small cords lie in art 
obliquely transverse situation, and form a 
kind of net-work at different distances-. 
The cavities of the inner surface of the. 
ventricles are small deep fossula; or lacuna; 
placed very near each other, with small pro- 
minent interstices between them. The great- 
est part of these lacunae are orifices of the ve- 
nal duct, to be described hereafter. 
Structure of the ventricles. 'Fhe fleshy 
or muscular fibres of which the heart is made 
up, are disposed in a very singular manner, 
especially those of the right or anterior ven- 
tricle, being either bent into arches or folded 
into angles. 
The fibres which are folded into angles are 
longer than those which are only bent into 
arches. The middle of these arches and the 
angles of the folds are turned toward the apex' 
of the heart, and the extremities of the fibres 
towards the basis.. These fibres differ not 
only in length ; but in their directions, which 
are very oblique in all, but much more so in. 
the long or folded fibres than in the short 
ones-which are simply bent. 
It is commonly said, that this obliquity re- 
presents the figure 8 ; but the comparison is 
very false, and can only agree to some bad 
drawings made by persons ignorant of the laws 
of perspective. 
Ail these fibres, regard being had to their 
different obliquity and length, are disposed 
in such a manner as that the longest form 
partly the most external strata of the convex 
side 'of the heart, and partly the most inter- 
nal on the concave side, the middle of the 
arches and the angles meeting obliquely and 
successively to form the apex. 
The fibres situated within these long ones, 
grow gradually shorter and straighter all the 
way to the basis of the heart, where they are 
very short and very little incurvated. By 
this disposition, the sides of the ventricles are 
