Gravid Uterus.] ANA" 
perly-defined (hape, and of which one part could not be 
dillinguilhed from another. There was, however, in that 
gluten a heart, which was the caufe of life and motion; 
there were veflels which generated the humour of the am¬ 
nios; there were therefore veflels of the umbilicus, the 
little trunks of which, being received from the foetus, are 
at that time very large. There was a head and fpinal co¬ 
lumn, bearing a larger proportion to the other parts of 
the body than they do afterwards. There were likewdfe, 
without doubt, all the reft of the vifcera, but, being pel¬ 
lucid and of a mucous nature, they were not lo apparent 
as they would have been had they been more opaque and 
folid. But in the whole foetus, an immenfe quantity of 
water is mixed together with a very little earth, as the 
cellular texture furrounds it in a ftate between fluid and 
folid, having large drops of water interpofed between the 
particles of the folid parts. To this vivifying gluten, or 
white of the egg, which is of the nature of lymph, there 
is added in birds a yolk, which is of an oily nature: in 
man fomething of a milky nature, not altogether unlike 
the yolk of an egg, is added to the coagulable lymph. 
That the blood is perfected from the fat by the proper 
powers of the foetus, we are perfuaded from the example 
of birds. From it are infenfibly prepared all the other 
humours; but all of them at firft mild, glutinous, void 
of tafte, colour, and fmell. It is long before they acquire 
their peculiar nature and properties, and fome of them 
are not produced till many years after birth, for inftance 
the femen. 
The veflels are the earlieft or oldeft parts of the bo¬ 
dy, and are prepared in the original delineation of the 
embryo. What firft appears in an egg during the time of 
incubation, having any dillind form, are venous circles: 
and thefe veins produce the arteries, by which they both 
receive their juice, and the motion of that juice. They 
are not generated mechanically from an obftacle againft 
which the arterial blood ftrikes. At firft the trunks of 
the veins are confpicuous, afterwards the branches which 
convey the humours to the trunks. If they were produ¬ 
ced from the arteries reflected, the branches would firft 
be feen, and the trunks would be formed laft. Neither 
could the arterial blood, driven back by an obftacle, form 
thofe mod beautiful circles, and bring back the veflels in¬ 
to the heart. It would rather flow irregularly through 
the cellular texture. And the primeval heart would foon 
lofe its life, unlefs as much of the humours returned to 
the heart as was fufficient to keep up its pulfations. 
There are, therefore, in the primeval foetus, fuch 
as we firft obferve it, fome parts more perfetft and con¬ 
fpicuous; and others involved, invifible, and very fmall. 
The heart is the moil perfect; it is the only movea¬ 
ble and irritable part; although it is in many refpeds 
different from what it is in an adult perfon. The brain 
is large and fluid; the veflels firft appear formed near the 
heart, and are vifible in the back. The vifcera, mnf- 
cles, nerves, and limbs, are not yet to be feen ; nor the 
bones, whole firft appearance is a mucus, nor the veflels 
of the reft of the body. The other vifible portion of the 
foetus,is the abdomen, of which the umbilical capfule is 
an immenfe hernia. 
To this embryo is fuperadded motion, in man almoft 
of the heart alone; as alfo in birds, whole formation does 
not take place without heat rather greater than that of the 
human body: yet, without the heart, heat deftroys, in- 
ftead of forming, the foetus. In the beginning the propor¬ 
tion of the heart to the reft of the body is the greateft; 
and is ever after continually growing lefs. Its pulfations 
are alfo at this time the mod frequent, and are very pow¬ 
erful for impelling the humours, and diftending and pro¬ 
ducing the veflels of the foft and tender foetus. The 
vifcidity of the vital humours which colled the earthy 
elements is oppofed to the force of the heart, and by that 
means the formation.of the foetus is aflifted. For there 
is in the embryo both an impelling force, which extends 
the parts longitudinally; and a refilling force, which mo- 
Vol. 1 . No, 41. 
r O M Y. 6 45 
derates the increafe, and regulates the lateral prelfure, and 
thus the diftention. By the force of the heart, the arte¬ 
ry, with all its furrounding cellular texture, is lengthen¬ 
ed ; its folds are ftretched, and the artery itlelf is dilated. 
The blood by its lateral prelfure makes an effort againft 
the almoft-blind branches of the arteries, fills and evolves 
them, and lets them oft'at more obtufe angles: thus are 
produced fpaces which make very little reftftance, in which 
the glutends depofited. In the very fubftance of the ar¬ 
tery itlelf, while it is every where dilated, between its.in¬ 
conceivably fmall folid threads, are prepared little reticu¬ 
lated fpaccs like a ftretched-out net, which are equally fit 
for receiving humours. The largeft of thefe are framed 
round the heart and in the head, whither the impufe of 
the heart drives the humours in a ftraight direction; and 
in the placenta: the fmaller ones are in the inferior parts 
of the body, whence the umbilical arteries fubtrad the 
greateft part of the blood. 
The foetus increafes very quickly, as is moft evident in 
the example of a chicken, whofe length the tvventy-le- 
cond day is to its length the firft day at leaft as 1,000,000 
is to 1; and the whole increafe of bulk in the bird during 
the remainder of its life does not exceed the fifth part of 
the increafe of the egg the firft day. For the feetus has 
a larger and more irritable heart, veflels larger in propor¬ 
tion, and likewife more numerous and relaxed, and the 
folid parts are mucous and diftenfible. The bread is later 
of coming to 'perfection, being furrounded with mem¬ 
branes fo fine that they cannot be feen. The embryo not 
only increafes in bulk, but is fo remarkably altered in 
ffiape, as to be brought forth totally unlike the appearance 
it had at firft. It is probable, that the limbs are produced 
from the elongated arteries; that they are laterally knit 
together by a certain gluten; that they are feparately 
evolved; that at firft they fprout out very ftiort, but af¬ 
terwards increafe by infenlible degrees, and appear divi¬ 
ded into dillind articulations; as the wings of a butter¬ 
fly are formed from vafcular net-work. Thus likewile 
the right ventricle of the heart is expanded by the blood 
coming to it in greater quantity ; and, being increafed by 
degrees, equals the left. 
On the other hand, the cellular texture, from its gluti¬ 
nous aqueous nature, by earthy particles being continually 
brought to it, becoming infenfibly harder, by a gentle 
attraction contracts its parts, which were before ftraight, 
into various flexures; and ties the auricles to the heart 
from which they were as yet at a diftance. So the muf- 
cles draw out procefles from the bones by their continual 
pulling, and open fmall cayities into large cells: the fame 
likewife incurvate the bonese and give them different 
fliapes. Prelfure alfo can doa great deal; to it we muff 
attribute the delcent of the tefticles into the ferotum, af¬ 
ter the irritable force of the abdominal mufcles has taken 
place; to this alfo we muff aferibe the repulfioa of the 
heart into the breaft, when the integuments of the bread 
are larger; to it we are to aferibe the length of the breaft 
and the ffiortnefs of the abdomen, and the fmaller fize 
of the vifcera of the latter; .becaufe the air received 
into the lungs dilates the cavity of the thorax. But even 
the bones are varioufly hollowed out by the prelfure of 
the mufcles, blood-veflels, and even of the very foft brain 
itfelf; and by the fame means flelh is changed into a ten¬ 
dinous fubftance. 
The power of derivation brings the blopd into the pel¬ 
vis and lower extremities from the clofed umbilical arte¬ 
ries : this fame power, when the foramen ovale is con- 
traded by the auricles being drawn towards the heart, 
evolves the right ventricle of the heart. So when the 
veflels of the yolk nave taken up the whole length of the 
egg, and can receive no farther elongation, it dilates the 
umbilical arteries of the chick, and produces a new mem¬ 
brane with incredible celerity. On the other hand, but 
ffill by the fame power, after the blood has got an eafy 
paflage through fome veflels of any part, the other parts 
which do not afford a like eafy paffage increafe the lefs. 
8 B Thus 
