A N A T O M Y. 6r 
indented notch or finus oppofite to the apex of the heart, 
which is therefore never covered by that lung, even in the 
ftrongeft inspirations; and consequently the apex of tiie 
heart and pericardium may always Strike againft the ribs ; 
the lungs not Surrounding the heart in the manner com¬ 
monly taught. This Sinus is exprelTed in Eullachius’s 
Tables. 
The veffels which compofe part of the SubStance of the 
lungs are of three or four kinds; the air-veffels, blood- 
veffels, and lymphatics, to which we may add the nerves. 
The air-velfels make the chief part,, and are termed bron¬ 
chia. Thefe bronchia are conical tubes, compofed of an 
infinite number of cartilaginous fragments, like So many 
irregular arches or circles, connected together by a liga¬ 
mentary elaftic membrane, and difpofed in Such a manner 
as that the lower ealily inlinuate them Selves within thofe 
above them. Tlte bronchia are alfo divided, in all direc¬ 
tions, into an infinite number of ramifications, which di- 
minifh gradually in Size; and, as they become capillary, 
change their cartilaginous ftructure into that of a mem¬ 
brane. Befides thefe very fmall extremities of this nu¬ 
merous Series of ramifications, we find that all the fubor- 
dinate trunks, from the greateft to the fmallefl, fend out 
from all Sides a vaft number of Short capillary tubes of 
the fame kind. Each of thefe tubes is widened at the 
extremity, and thereby formed into a fmall membranous 
cell, commonly called a vfcle. Thefe fmall veficular 
cells are formed into bundles, termed lobules ; and, as the 
great branches are divided" into Small rami, fo the great 
lobules are divided into Several fmall ones. All the bron¬ 
chial cells are Surrounded by a very fine reticular texture 
of the fmall extremities of arteries and veins, which com¬ 
municate every way with each other. The greafeft part 
of this admirable ftrufture is thedifcovery of the illustri¬ 
ous Malpighi. 
The blood-veffels of the lungs are of two kinds; one 
common, called the pulmonary arteries and veins ; the other 
proper, called the bronchia?. arteries and veins. The pul¬ 
monary artery goes out from ti e right ventricle of the 
heart; and its trunk, having run directly upward as high 
as the curvature of the aorta, is divided into two lateral 
branches ; one going to the right, called the right pulmo¬ 
nary artery ; the other to the left, termed the left pulmo¬ 
nary artery. Befides thefe capital blood-veffels, there are 
others called the bronchial arteries and veins, wnich are ve¬ 
ry Small, but they follow the bronchia through all their 
ramifications. They communicate with the pulmonary 
arteries and veins in many places; and likewife with the 
arteries and veins of the cefophagus, and with the branch¬ 
es of the coronary artery and vein. 
The principal ufe of the lungs is for refpiration, in 
which they are chiefly alfifted by the diaphragm and inter- 
cofial mufcles. As Soon as the intercoftal mufcles begin 
to contrafl, the arches of the ribs are railed together with 
the fternum, and placed at a greater diftance from each 
other; by which means the cavity of the thorax is enlar¬ 
ged on the two lateral and anterior Sides. At the Same 
inftant the diaphragm is flatted or brought toward a plane 
by two motions, which are apparently contrary ; that is, 
by the contraction of the diaphragm, and dilatation of the 
ribs in which it is inferted. The external Surface of the 
thorax being thus in a manner incrqafed, and the cavity 
of the bronchia being at the fame time, and by the fame 
mean, lefs refilled or prelfed upon, the ambient air yields 
to the external preffure, and infinuates itfelf into all the 
places where the preffure is diminiftied; that is, into the 
afpera arteria, and into all the ramifications of the bron¬ 
chia, all the way to the vehicles. This is what is called 
infpii ation. This motion of infpiration is inftantaneous, 
and ceafes in a moment, by the relaxation of the inter¬ 
coftal mufcles; the el iltic ligaments and cartilages of the 
ribs bringing them back at the lame time to their former 
Situation. This motion, by which the ribs are deprelfed 
and brought nearer to each other, is termed expiration. 
Tlie pulmonary arteries and veins, which accompany the 
bronchia through all their ramifications, and furround the 
vehicles, tranfmit the blood through their narrow capil¬ 
lary extremities, and thereby change cr modify it, at lead 
in three different manners. The firfi change or modifica¬ 
tion which the blood undergoes in the lungs, is to have 
the cohefion of its parts broken, to be attenuated, and, 
as it were, reduced to powder. The fecond is, to be de¬ 
prived of a certain quantity of ferum, which tranfpires 
through the lungs, and is what we commonly call the 
breath. The third is, to be in a manner re-animated by 
the impreflionof the air, whether the whole body of the 
air enters the blood, whether the common air is only the 
vehicle of fome finer parts which are conveyed to it, or 
whether the air only compreffes and lhakes tlie blood as it 
paffes round the bronchial vehicles in the reticular capilla¬ 
ry extremities of the velfels. 
The diaphragm almofi alone performs the office of ref¬ 
piration in a healthy man at reft. The diaphragm is alfo 
the chief inftrument of refpiration in the cafe of fraifhired 
ribs or fternum, or where the ribs cannot be moved with¬ 
out exciting confiderable pain. The force of the dia¬ 
phragm alfo, in dilating the breaft, is greater, according 
to the calculations that have been made, than all the reft 
of Lie powers which contribute to refpiration. A great 
infpiration is fomewhat hindered; becaule, during the 
greateft exertion of the diaphragm, the lowermoft ribs are 
brought inwards, and tints the thorax is ftraightened. 
Left this fliould always happen, the intercoftal mufcles in¬ 
terfere in ordinary infpirations ; in the very great ones, 
they are inferior to the diaphragm. The phrenic nerve, 
which is more ealily irritated than in mod other mufcles, 
forces the diaphragm to perform its office. The lungs 
themfelves are altogether patfive or obedient to the adiion 
of the air; and alio to that of the ribs and diaphragm, 
to which they are preffed into clofe contact on all fides. 
But refpiration, whether-by the admixture of a fob- 
putrid vapour, or by fome other method, certainly vitiates 
the air, and renders it unfit either for in Rating the lungs 
or fupporting flame; and laflly, it deprives that element 
of its elafticity. We may fuppofe that this happens from 
putrefaction, fir.ee the air is rendered peftilential by a 
crowd, and fevers of the moft malignant kind are thus ge¬ 
nerated in a few hours. But, in whatever manner:this is 
produced, we are certain that the air is vitiated in the 
lungs, lofes its elafticity, and thus cannot keep the lungs 
diftended, fo as to tranfmit an inertafed quantity of blood 
through the dilated pulmonary arteries into the veins. 
Nor can the will dilate the breaft beyond certain bounds,- 
or afiift that paffage of the blood in an unlimited manner, 
A flate of body, therefore, will take place, in which the 
blood cannot pafs through the lungs. Thus is generated 
a new refiftance to the blood continually coming from the 
heart. And in long retentions of the breath, as in ma¬ 
king violent efforts, the venous blood, efpecially that co¬ 
ming from the head, ftagnates before the right ventricle 
of the heart, which is fliut, becaufe it cannot evacuate it¬ 
felf into the lungs, and thus fvvells up the face with red- 
nefs, fometimes burfts the veins of the brain, neck, intef- 
tines, kidneys, and laftly of the lungs and even right 
auricle cf the heart. This ftagnation of the blood occa- 
fions prodigious anxiety and uneafinefs to the fpirits; it is 
alfo the caufe of death in compreffed air, in drowned peo¬ 
ple, and fuch as are ftrangled, which is much more hidden 
than is commonly imagined. A living perfon, therefore, 
that he may remove thofe inconveniences which proceed 
from an obftrudlion of the paffage of the blood, llackens 
the powers of infpiration, and excites thofe of expiration, 
which free the breaft from an air too greatly rarefied. 
Thefe powers are, at nrft, the elafticity of the ribs ; which 
being drawn upwards out of their natural fituation, as 
loon as the powers which elevated them ceafe to a£l, fpon- 
tarreoufly place themfelves, fo as to make more acute an¬ 
gles with tlie fternum and vertebrae. To this end conduces 
likewife the elaftic force of the bronchia and veficles dil- 
tended with air, which firive to contrail themfelves. 
Hencjg 
