ANATOMY. 
fufceptibility of the aftion of ftimuli, upon which the 
commencement and continuance of life depends. 
The brain, or that mafs which fills the cavity of the 
cranium, is at once the origin and point of union to the 
whole nervous fyftem. This general mafs is divided into 
three particular portions, the cerebrum, the cerebellum, 
and the medulla oblongata ; and a continuation of the 
latter forms the medulla fpinalis, which fills the cavity of 
the vertebrae. This organ, and the various branches de¬ 
rived from it, form one of the mod important parts of the 
animal oeconomy. It is efl'ential to all the nobler fpecies 
of animals; but its importance is more peculiarly evident 
in man than in any other. In the human race, it bears 
a much larger proportion to the fize of the animal than 
in any of the inferior orders. The nerves, which form 
the various organs of fenfation, and upon which ulti¬ 
mately the aftion of mufcular fibre depends, are derived 
from the brain in the inferior animals, as well as in man ; 
but, from the much larger iize of this organ in the lat¬ 
ter, it appears deftined to perforin fome other and more 
important purpofes'in him than in the former. The 
whole of the nerves, it has been obferved, bear but a 
fmall proportion to the mafs of medullary fubftance con¬ 
tained in the brain, being at lead an hundred times as 
large as the diameters of all the nerves of the head and 
fpinal marrow. It does not appear that the large fize of 
the brain gives to man more acute fenfations, or a greater 
power to produce mufcular motion, than other animals 
polfefs in which the brain is proportionally much lefs. 
Thus, the brain of a large ox has been found to weigh 
not more than one fourth part of the human brain, whilft 
the weight of the ox was probably fix times greater than 
that of the man ; or the brain of the man was, in propor¬ 
tion to his weight, twenty-four time's heavier than that of 
the ox. At the fame time, the nerves of the mufcles of 
an ox are in their lize proportioned to the bulk of the ani¬ 
mal’s” mufcles, and thofe of the organs of fenfation, as 
the eye and nofe, are proportioned to the extent of thefe 
organs. Thus the olfactory nerve of an ox is many times 
larger than that of a man. Again we oblerve, that three 
fourths of the nerves originate from the medulla fpinalis ; 
yet it is only a fmall part of the brain which is elongated, 
and palfes down the vertebral canal. We have reafon 
therefore to conclude, that to give origin to the nerves is 
not the l'ole ufe of the brain: it is the feat of intellect; 
the medium by which impreffions made on the organs of 
fenfation are conveyed to the mind. 
The whole of the nerves, then, arife from one common 
fource; and the brain mull be conlidered as the central 
point, at which they all unite. Arifing from this point 
they are tranfmitted to every part of the body, they are 
interwoven in every organ of fenfation, every mufcle, 
veil'd, and bone, and thus unite the various parts fo as 
to conftitute an uniform fyfiem. Poffeiled of a peculiar 
organization, by which they are highly fufoeptible of im- 
preflions from various ftimuli, they receive impreffions on 
every part, which appear to be inftantaneouily tranfmitted 
to the central point, the cenforium commune. Hence it 
is, that the action of a ftimulant upon a particular part 
does not affeCt that part only to which it is applied, but 
operates on a greater or fmaller degree upon the whole 
fyfiem. Minute anatomy evidences, by an infinite variety 
of Circumftances, the peculiar fitnefs of the brain and ner¬ 
vous fyfiem for producing thefe effects. We have already 
obferved, that the brain is formed of three portions, the 
cerebrum, cerebellum, and medulla oblongata, to which 
a fourth may be added, the medulla fpinalis. The whole 
of thefe are covered by their peculiar membranes, the pia 
mater and dura mater. Thefe ferve for their protection, 
and to convey blood-veffels for their nourilhment and fup- 
port. The brain is fupplied with numerous blood-veffels 
from the carotid and vertebral arteries; and, by the many 
convolutions which they make before they pafs through 
the dura mater, and the vaft number of communicating 
branches into which they are divided in the pia mater, 
53 » 
and its proceffes, it appears, that the blood mufi move 
more flowly and equally in thofe veffels than in thole of 
other parts. We may alfo add, that the arteries in other 
parts are in fome degree aCted upon by the labouring 
mufcles, and the preffure of the atmofphere, by which the 
blood is propelled with additional force. But the arteries 
which fupply the brain, after they enter the cranium, are 
not fubjeCted to the operation of thefe powers. We ob- 
ferve then, in this distribution of the blood-veffels of the 
brain, a provision made againft thofe injuries which mult 
have infallibly taken place in fo delicate an organ, if it 
had been pofiible for the blood to have been driven into 
it with great violence. A very considerable quantity of 
blood is tranfmitted to the brain; it is derived from trunks 
which arife near the heart; and, according to the opinion 
of Haller, a fixth part, or, as Dr. Monro luppofes, not 
lei's' than a tenth part, of the circulating mafs, is tranfmit¬ 
ted to the brain. From the ample fupply of this fluid 
which the brain receives, it appears that perpetual and 
important changes take place in the fubftance of the brain 
and nerves, as well as in other parts. 
The cerebrum, cerebellum, and fpinal marrow, are na¬ 
turally divided into the external part, the cortex or cine- 
ritious fubftance, and the internal part, or medullary mat¬ 
ter. The minute branches of the blood-veffels, by the 
affiftance of injections and the microfcope, are found to 
pal's from the pia mater into the cortical part in vaft num¬ 
bers ; in faCt, it appears to be almoft wholly formed of 
veffels ; but into the medullary fubftance wc only obferve 
longitudinal veffels entering. The veins which return 
the blood from the brain are of a peculiar ftrudture, evi¬ 
dently deligned to facilitate that return. Their coats are 
of peculiar ftrength: they form innumerable anaftamoles 
with one another; and, when collected fo as to form con- 
fiderable trunks, they are lodged in canals, which prevent 
them from being i'ubjeCted to compreffion. A proviiion 
therefore is evidently made to prevent an injurious accu¬ 
mulation of the blood in this organ. It has been doubted, 
whether the brain is fupplied with lymphatics ? They 
may certainly be demonstrated upon the pia mater ; and 
from analogy drawn from every other part of the body, 
we fcarcely luppofe that the brain is deftitute of lympha¬ 
tics. Indeed modern phyfiologifts in general admit the 
exiftence of lymphatics in the brain. 
From the blood-veffels, as we have already hinted, the 
cortical part of the brain receives its origin. Anatomical 
injections fully prove, that the greater part of it confifts 
of veffels which are inferted from the fmall branches of 
the pia mater. Thefe veffels, however, in the natural 
ftate, do not convey red blood, but a thinner fluid ; al¬ 
though in fome dileafes, and in death by ftrangling, par¬ 
ticularly in brutes and birds, the red blood is propelled 
into them. No anatomift has, indeed, Succeeded in filling 
every portion of the cortex with injected matter ; yet it is 
probable, that it is altogether vafcular. We obferve no 
diffimilarity of parts in it when in the entire or natural 
ftate, fo that we cannot imagine that it is partly vafcular 
and partly folid. 
As the cortical fubftances originate from the veffels of 
the pia mater, fo the white medullary matter is derived 
from the cortex. The continuity of the medulla with the 
cortex, is diftinftly feen by the microfcope, and may even 
be obferved by the naked eye. Minute portions of the 
medulla may be obferved proceeding from the cortex; 
and its gradual increafe may be traced, becoming more 
broad and abundant till it makes up the whole oval feCtion 
of the brain, and is bounded only by the convolutions of 
the cortex. The medulla is extremely loft, and of a fibrous 
texture. The fibres are vifible in feveral parts of the hit¬ 
man brain, and ftill more evidently in the brains of fifties, 
efpecially in their thalami optici. The fibres of the me¬ 
dullary fubftance are (imply extended, fo as to form the 
white fibrous cords which are diftributed to every part of 
the body, and which are univerfally termed the nerves. 
That the fibres of the nerves are merely a continuation 
