65S A N A 1 
^vjhe vena cava of a confiderable length within the thorax, 
having near the whole length of the heart to run over ere 
it gets at the fmus Lowerianus dexter. In men it enters the 
pericardium as foon as it pierces the diaphragm, which is 
firmly attached to it, and immediately gets into the fmus 
Lowerianus; which fmus, in the human fubject, by the 
.obliqued! tuation of the heart, is almoft contiguous to the 
diaphragm: and by this we difeover, that feveral authors 
have taken their delineations of the human heart from 
brutes; which is eafily detected by the lhape and fituation 
of the heart, and the long vena cava, within the thorax. 
- This fituation of the heart of the dog agrees beft with 
the fliape of its thorax, which is lower than the abdomen. 
The egrefs of the large blood-velfels from the heart is 
fomewhat different from the human ; for here the right 
fubclavian comes off firft; and as a large trunk runs fome 
way upwards before it gives off the left carotid, and fplits 
into the carotid and fubclavian of the right fide, then the 
left fubclavian is fent off. So that, neither here, properly 
fpeaking, is there an aorta afeendens more than in the hu¬ 
man; but this name has probably been impofed upon it 
from obferving this in a cow, where indeed there is an af- 
cending and descending aorta. From this lpecialty of the 
ditfribution of the vellels of the right fide, which hap¬ 
pens, though not in fo great a degree, in the human fub- 
je« 5 f, we may perhaps in fome meafure account for the ge¬ 
neral greater ftrength, readinefs, or facility of motion, 
which is obfervable in the right arm. We believe, up¬ 
on meafuring the fiaes of the veffeis, the furface of the 
united trunk of the right fubclavian and carotid is lefs 
than that of the left fubclavian and carotid, as they 
are Separated. If fo, the refiflance to the blood mult be 
lefs in that common trunk than in the left fubclavian and 
carotid. But, if the refinance be fmaller, the abfolute 
force with which the blood is fent from the heart being 
equal, there muff neceffarily be a greater quantity of blood 
fent through them in a given time ; and, as the ftrength 
of the mulcles is, caleris paribus , as the quantity of blood 
fent into them in a given time, thofe of the right arm 
will be ftronger than thofe of the left. Now children, 
being conlcious of this fuperior ftrength, life the right 
upon all occafions; and thus from ufe comes that great 
difference which is fo obfervable. But, if by any accident 
the right arm is kept from aftion for fome time, the-other 
-from being ufed gets the better; and thofe people are left- 
handed. For it is not to be imagined, that the fmall odds 
in the original formation of the vellels Ihould be fufiicient 
to refift the effedf of ufe and habit, (inftances of the con¬ 
trary occur every day ;) it is enough for our prefent argu¬ 
ment, that, where no means are tiled tooppofe it, the odds 
are fufficient to determine the choice in favour of the 
right. Now, becaufe it is natural to begin with the leg 
correfponding to the hand we have moll power of, this is 
what, gives alfo fuperiority to the right leg. This diffe¬ 
rence is not peculiar to man, but is ftill more obfervable 
in thofe creatures in whom the fame mechanifm obtains in 
a greater degree. Obferve a dog at a trot, how he bears 
forward with his right fide; or look at him when feraping 
up any thing, and we (hall prefently fee that he ufes his 
right much oftener than his left foot. Something analo¬ 
gous to this may be obferved in horfes; who are never 
• confidered to gallop well, unlefs the right leg be prefented 
firft. It has been the opinion of fome anatomifts, that 
left-handed people, as well as thofe diftinguilhed by the 
name of ambidexter, (who ufe both hands promifeuoufly,) 
have the two carotid and fubclavian arteries coming off'in 
four diftindt trunks from the arch of the aorta; but no 
appearance of this kind has ever been obferved in fuch 
bodies as have been examined for this purpofe; though 
indeed thefe have been but few, and more experience 
might throw greater light on the fubjedl. 
The thymus of the dog is proportionally much larger 
than ours; whereas the glandula thyroidea is much lefs, 
and is divided into two diftinft parts, or there are two fe¬ 
rrate glands ; which is not the cafe in man. The reafon 
O M Y. ["Comparative. 
of this difference is unknown, as is likewife the ufe of 
the gland itfelf. It is generally remarked, that thefe two 
glands fupply the place of each other; that is, in fuch 
animals as have a large thymus, the glandula thyroidea 
is fmaller, and vice verja. Hence we are naturally led to 
aferibe the fame ufe to both, viz. the reparation of a thin 
lymph for diluting the chyle in the thoracic duct before it 
be poured into the blood; then, if we conllder the diffe¬ 
rent formation of the thorax in both, we fhall readily ac¬ 
count for the variety in the bulk of thefe two glands. 
Refpiration being chiefly performed in man by the widen¬ 
ing of the cheft, the lungs at every infpiration muft prefs 
upon the thymus, and confequently diminifh .it: but the 
diaphragm, yielding more in the dog’s infpiration, this 
gland is not fo much preffed by the lungs, and fo will be 
iarger; and hence the glandula thyroidea will be propor¬ 
tionally lefs. Again, from the pofture of this creature, 
we (hall fee that it was much more convenient for a dog 
to have the mod part of the diluting lymph fupplied by 
the thymus, fince, the neck being frequently in a defend¬ 
ing pofture, the lymph of the thyroid gland would have 
a very difadvantageous courfe to get to the thoracic dudt: 
whereas in the human body, the thymus is really below 
the lafteal canal, where it makes its curvature before it 
opens into the fubclavian; and confequently there is a ne- 
ceiTity of a confiderable (hare of the diluting liquor being 
furniftied by the thyroid gland, which is (ituated much 
higher ; fo that its lymph has the advantage of a perpen¬ 
dicular defeent. 
We may here obferve, that the thoracic duEl in a dog 
has no curvature before it enters the fubclavian vein, the 
horizontal polition of this animal allowing a favourable 
courfe to the chyle, fo as not to need that turn to force its 
pallage into the blood. It may likewife be obferved, that 
fuch animals as walk horizontally, have the valves of the 
thoracic du£t fewer in number than others. The horfe 
has only a fingle pair ; while, on the contrary, the ape re- 
fembles man in having feveral valves. Thus the lymph is 
forwarded in its pallage, and the weight of it diminifhed. 
The lungs of the dog are divided into more numerous 
lobes, and deeper, than they are in man, for the fame 
reafon as the liver. The left fide of the thorax in this 
animal bears a greater proportion to the right than in man ; 
the one being nearly as three to two, the other as four to 
three. In quadrupeds, as well as in man, the lungs are 
clofely applied to the containing parts; although this has 
been denied by fome authors. 
We conllder it as a general rule, that all quadrupeds, as 
having occalion to gather their food from the ground, are 
provided with longer necks than man: but as a long neck 
not only gives the advantage of too long a lever to the 
weight of the head, but alfo, w'hen the animal is gather¬ 
ing his food, makes the brain in danger of being oppreft- 
ed with too great a quantity of blood, by the liquor in 
thefe arteries having the advantage of a defeent, while 
that in the veins muft remount a confiderable way contrary 
to its own gravity; it was therefore necelfary that a part 
of the length of the neck Ihould be fupplied by the length 
of the jaws. Thus we fee horfes, cows, &c. who have 
no occalion for opening their mouths very wide, yet have 
long jaws. Bull-dogs indeed, and fuch animals as have 
occafion for very ftrong jaws, muft of necellity have them 
fhort; becaufe, the longer they are, the reliftance to be 
overcome a£fs with a longer lever. Another exception to 
this general rule is, fuch animals are furnilhed with fome- 
thing analogous to hands to convey their food to their 
mouths, as cats, apes, See. The teeth of the dog plainly 
fliew it to be of the carnivorous kind ; for there are none 
of them made for grinding the food, but only for tearing 
and dividing it. It has fix remarkable (harp teeth before, 
and two very long tulles behind ; both of which the rumi¬ 
nating animals want. Thefe are evidently calculated for 
laying very firm hold of fubllances, and tearing them to 
pieces; and the vaft ftrength of the nmfcles inferted into 
the lower jaw afliits greatly in this action; while the mo- 
lares 
