FARRIERY. 
This membrane is there double, but unites about an inch 
above the fterniml : and the fpace between the double 
refledtion is filled by cellular fubflance pofterio.rly, and 
by the thymus gland anteriorly. This fpace is called the 
inferior mediaftinum : the two lamina then le'pa.rating, 
pats over the pericardium, and over the veflels, to be re- 
fledfed on the lungs. After having palfed over the lungs; 
one portion goes over the fpine on one fide, the other por¬ 
tion over the lpine of the other fide ; leaving in this divi- 
fi'on a tubular opening, filled by the aoria, vena cava, 
thoracic duct, and cefophagus. This forms the fuperior 
mcdiaftinuin. There are therefore two reflections of the 
pleura on each fide ; one dole tinder the rib', the other 
in contadt with the lungs. A bag is formed by this re- 
fledtion ; but the lung is not contained within it, as is 
ufually defcribed, but a fma'tl quantity of water, the liquor 
pleura. The mediaftinum is (imply the jundtion of the 
two lamina of the pleura, dividing by diltindt partitions, 
the cavity of the cheft. 
At tIre anterior and inferior divifion of the pleura, is 
fituuted the thymus gland ; being under the aorta and 
vena cava, near to their firft divifions. It is compofed of 
two lobes of confiderable magnitude, and faintly red in 
the colt ; but much darker, and lefs in fize, in the grown 
horfe ; and fcarcely difcernible in old age. ft was thought 
to be larged in fize fome months after foaling ; but it is 
now proved to exift larger in the foetal colt previous to 
birth, than at any period afterwards. From this,, though 
its life is not fully underflood, it appears more eflential ro 
the exiflence of the animal previous to birth, than af¬ 
terwards, Jtisfupplied with veflels called arterias and 
venre thjfmicte, and with nerves from a neighbouring 
plexus. 
The lungs, when inflated, are faid to refemble the 
figure of the cloven foot of the ox. This vifeus in the 
horfe confifls of three lobes, two large ones, which occupy 
the tides of the cheft ; the third, or central lobe is much 
fmaller, lying between the other two, and prefe.nts itfelf 
to the fternum. They are fpongy in their texture, being 
every where full of little cavities, which are the minute 
ramifications, or dillrib.utio'n of velTels termed bronchial, 
ariling from one common trunk, the trachea or ivind-pipe. 
By means of thefe veii'els, air is received throughout the 
fubflance of the lungs; depofiting its vital part, it is t lien 
returned from thence and frefh air admitted. The lungs 
move freely within the cheft, or rather readily obey its 
motions. In this they are aftifted by their being divided 
into feparate portions; and further by the contained fluid. 
In both the living and dead ftate- they leave no fpace be¬ 
tween their furfaoes and that of the inner part of the 
cheft ; but are always diftended by the air to the capacity 
of the cavity. Their Functions, and particular ftrmSture, 
are more fully explained in our firft volume, under Ana- 
tout. Between the lungs is placed the heart, lupport¬ 
ed in its fituation by various attachments, and covered by 
the membrane called pericardium. 
The pericardium firft: loofely inverts the heart, then 
paftes clofe over its furface, giving it a fecond covering. 
Between the one reflection and the other is contained a 
quantity of water,called liquor pericardii. The peiicardiiuri 
where it loofely inverts the heart, is more denfe than the 
pleura. It has few arteries or veins, and apparently little 
lenfibility. Its internal furface is that which pours out 
tlie liquor pericardii. The heart itfelf is of an elliptical 
form: its anterior and fuperior part is called its bafis ; its 
pofterior and inferior part its apex. The bafis is confined 
by veflels ; the apex is loofe, and ftrikes againft the inter- 
coftal mufcles, giving thefe vibrations known by the 
name of pulfation. 
The heart is alfo divided into two cavities, termed 
ventricles , each of them having a fmall hollow appendage, 
which, from a (light refemblance it bears to a dog’s car, 
is named auricle. When the left ventricle is full of blood, 
it contracts fo powerfully as to force its contents into the 
aorta, or grand artery, by which the blood is diftributed 
239 
all over the body; it is then taken up by the veins, as 
before noticed, and conveyed to the right auricle, whence 
it flows into the right ventricle ; this alio, v\ hen it is fuf- 
ficiently diftended, contra&s upon its contents, and pro¬ 
pels the blood into the pulmonary artery, by which it is 
conveyed to every part of the lungs. The pulmonary 
veins then receive it, and convey it to the left auricle, 
from whence it is propelled into the left ventricle, that 
it may again be diftributed by the aorta to every member 
of the body. Thus the blood is continually circulating. 
If we examine the ftate of the blood while in the left 
ventricle of the heart, and in the arteries, it will be found 
of a bright red colour, and replete with thofe properties 
which render it capable of nourifning the body, and fti- 
mulating.the whole fyftem to life and action : but in the 
veins it becomes of a much darker colour, and when it 
arrives at the right ventricle, it is nearly black, and defti- 
tute of thofe vivifying qualities which it poflefled when 
in the left ventricle: had not Providence therefore pro¬ 
vided fome means for its renovation, it would have been 
quite unfit for a fecond circulation, and the duration of 
life mnft have been fhort indeed ; but from the right ven¬ 
tricle it.is conveyed by the pulmonary artery to the lungs,, 
at the moment they are diftended with air ; here the blood 
undergoes a wonderful purification; in proportion, per¬ 
haps, as the air is pure, and free from azotic gas : it now 
recovers its bright fcarlet colour, and is then returned by 
the pulmonary veins to the left fide of the heart, with all 
its original and eflential qualities reftored to it, for the 
renovation of health and life. 
The trachea of the horfe, where it paftes into the cheft, 
and previous to its entering the lungs, poft'elfes a lingular 
duplicature of the. cartilaginous rings, which is more 
flexile than the reft of this tube, fo as to admit, by very 
flight prerture, the (ides of the tube to be brought in con-. 
ta£t, and the total obliteration of its cavity .- this ftruc- 
ture may perhaps adapt the diameter of the trachea to 
any quantity of air that is paffing through it to the lungs. 
The Farriery Plate IX. at fig. j, exhibits a correct re- 
prefentation of the heart and lungs, as placed in their 
natural polition in the fecond cavity or cheft ; (hew ing 
likewil'e the cellular covering or bronchial veflels of the 
lungs, as ramifying over them from the duplicature of 
the trachea on its entering the cheft. The polition of the* 
heart is (hewn as embraced by the two great lobes of the 
lungs; and its union or connection with the aorta,, vena 
cava, pulmonary artery, &c. is corrcdtly delineated in the 
Farriery Plates IV. and VII. to which the reader is re¬ 
ferred. Fig. 2, in the-annexed engraving, (hews the right 
ventricle of the heart laid open : a, the interior fibres ; 
b, the anfracluofities; c, the tricufpidal valvules ;, d, the 
fygmoid valves; e, the union of the feotion with the left 
ventricle. Fig. 3, a vertical feifion of the heart, exhi¬ 
biting the interior lining of both the ventricles, and of 
the right auricle : a, the tricufpidal valvules of the left 
ventricle; b, aperture of the pulmonary artery ; c, aper¬ 
ture of the aorta ; d, .anterior aorta ; c, the right auricle- 
laid open ; f, tricufpidal valvules of the right ventricle.— 
For the pulfation of the heart, and circulation of the 
blood, fee the article Anatomy, vol. i. p. 605-610. 
Of the ABDOMEN or BELLY. 
The abdomen, or cavity of the belly, is much the 
large!! of the three, forming, when its contents are re-, 
moved, an extenfive oval vault; bounded anteriorly by 
the diaphram, pofteriorly by the bones of the pelvis, 
luperiorly by the vertebrae’, laterally by the ribs, and in¬ 
ferior! y by the abdominal mufcles ; fo that the fuperior 
and anterior part is generally called the back, and pofte¬ 
rioriy the loins; the lateral parts the Tides, more pofte¬ 
rioriy the flanks; and the inferior portion throughout its 
whole length, the belly. But thefe divifions being more 
properly exterior, and withal too indeterminate ; it was 
neceflary to make others, that the fituation of the vifeera^ 
or the peculiar part of the abdomen affe&ed under difeafe* 
roigLt 
