ANATOMY, 
594 
to be very diftindtly feparated from the internal lamina, in 
the interfaces. between all thefe eminences on the lower 
iide of the medulla oblongata. From this common por¬ 
tion of the cerebrum and.cerebellum, arife almoft all the 
nerves which go out of the cranium, through the different 
foramina perforating its bafe. It likewife produces the 
medulla fpinalis ; and may juftly be faid to be the firft 
origin or primitive fource of all the nerves which go out 
through the fpina dorfi, and confequently of all the nerves 
of the human body. 
The medulla fpinalis is only an elongation of the extre¬ 
mity of the medulla oblongata; and it has its name from 
its being contained in the bony canal of the fpina dorfi. It 
is confequently a continuation or common appendix of the 
cerebrum and cerebellum, as well becaufe of the two fub- 
itances of which it is compofed, as becaufe of the mem¬ 
branes by which it is inverted. The fpinal marrow con- 
lifts of a cortical and medullary fubftance, as the cerebrum 
and cerebellum; but with this difference, that the alh- 
coloured fubftance lies within the other; and, in a tranf- 
verfe fedlion of this medulla, the inner fubftance appears 
«o be of the form of a horfe-fhoe, the convex fide being 
turned forward and the extremities backward. The body 
of the medulla fpinalis runs down to the bottom of the 
firft vertebra of the loins, where it terminates in a point. 
The fize of it is proportionable to that of the bony canal, 
fo that it is larger in the vertebrae of the neck than in 
thofe of the back. 
From a general view’ of the nerves and blood-veffels be¬ 
longing to the brain, it appears that a very great quantity 
of blood, derived from trunks that are near the heart, is, 
at every puliation, fent to this organ: Dr. Haller fays a 
fixth part, Dr. Monro, a tenth part of the whole circulat¬ 
ing mafs. Hence it is probable, that the ftrongeft parts 
of the blood, and i’uch as are moft retentive of motion, go 
to tl'.e head. Is not this evident from the effects of mer¬ 
curials exerting themfelves almoft in the head only ? from 
the fudden force and adlion of inebriating fpirits upon the 
head? from the fiiort ftupor which camphor excites? from 
the heat, rednefs, and fweat, which happen oftener in the 
face than other parts of the body ? to which add, the more 
eafy eruption of volatile and contagious puftules in the 
face ? Dr. Wrilberg, however, oblerves, that all thefe 
arguments are not of the fame force; for mercury, ap¬ 
plied in different Ways to the body, produces its effects not 
in the head alone ; fince it occalions in fome patients a dia- 
phorefis, in others a diarrhoea, and in others it adts as a 
diuretic. 
The veins of the brain are not difpofed in the fame man¬ 
ner with thofe in other parts of the body. For they have 
no valves, nor do they run together in company with the- 
arteries, nor have their trunks the ftrudture which is com¬ 
monly obferved in the other veins. The great quantity of 
blood which goes to the brain, the greater impulfe with 
which it is fent into the carotid arteries, the fecurity of 
this part from every kind of prelfure by a ftrong bony 
fence, the flower motion of the blood through the abdo¬ 
minal vifcera and lower extremities, and the perpetual 
exercife of the brain and fenfes, all determine a copious 
flux of blood to thefe parts; fome other caufes alfo ferve 
to fill the head largely with blood. Hence it is that a red- 
nefs of the face, a turgefcence, a fparkling of the eyes, 
with a pain of the head, a pulfation or throbbing of the 
arteries, and a bleed ng at the nofe, are produced by vio¬ 
lent exercifes or motions of the body. Hence, therefore, 
it is evident, that if the veins were of a thin ftruCdure, 
and round fhape, in the brain, they would unavoidably be 
in greater danger in rupturing, (to which, even in their 
prefent ftate, they are often liable,) and confequently apo¬ 
plexies would be much more frequent. To avoid this in¬ 
convenience, therefore, nature has given a different figure 
to the veins carrying the blood from the brain, by which 
they are more eafily and largely dilatable, becaufe they 
make an unequal refiftance : their texture is likewife very 
firm, and more difficultly ruptured, efpecialiy in the larger 
finufes, which perform the office of trunks; but, as to the, 
finufes ot the leffer fort, they are either round, half cy¬ 
lindrical, or of an irregular figure. Befides this, nature 
has guarded the finufes by crofs beams, or fraena, inter¬ 
nally, made of ftrong membranes, and detached from the 
right to the left fide of the bottom of the linus, which, 
in greater diftenfions, they draw towards a more acute 
angle, ftrengthening and guarding it from a rupture. She 
has likewife furnilhed thefe veins with numerous tnofcu- 
lations, by which they communicate with one another, 
with the external veins of the head, and with thofe of the 
fpinal marrow ; and by this contrivance, they are capable 
ot evacuating themfelves more eafily whenever they are 
overcharged with blood. 
Some writers have doubted, Whether a part of the ar¬ 
terial blood is not poured into the finufes of the brain; 
and whether the finufes have not a pulfation exc.ted from 
that blood? That they have no pulfation, Dr. Haller fays, 
is paft a doubt; becaufe the dura mater every where ad¬ 
heres firmly to the Ikull, but much more firmly in iliofe 
parts which are the feats of the finufes. But Dr. Monro 
obferves, that whilft the heart is performing its fyftole, the 
arteries here, as ellewhere, may be dilating ; and, in the 
mean time, a quantity of blood, equal to that which is 
dilating them, is paffmg out of the head by the veins. In¬ 
deed the finufes receive liquors injedted by the arteries ; 
but whether they tranfude through the fmall exhaling ar¬ 
terial veffels, or whether they firft make a complete circle 
through the veins, as indeed is much more probable, we 
are not yet furniihed with experiments enough to deter¬ 
mine. The veins form innumerable anaftomofes with one 
another, that the blood may return with the greateft eafe 
from the head, the repletion of which is very dangerous. 
The brain is alfo more eafily evacuated in the time of in- 
fpiration, for it then fubfides, as we fee when the fkull is 
opened, but fwells during the time of expiration. Flence, 
blowing the nofe, fneezing, and coughing, are dangerous 
to thofe whofe brain is fwelled by retained blood. 
Whether lymphatic veffels are to be feen in the brain, 
is queftioned by fome writers. Dr. Haller thought it pro¬ 
bable that no lymphatic veffels are in the brain, becaufe no 
conglobate glands are found there; and that the fuperflu- 
ous moifture is abforbed by the red veins. But, according 
to moft of the prefent phyliologifts, lymphatic veffels take 
up the fuperfluous fluids of the brain, as well as of the 
other parts of the body, although they cannot be diftindtly 
fhewn. “ Scarcely in any vilcus,” fays Dr. Wrilberg, 
“ has the exiftence of lymphatic veffels been oftener af- 
ferted, and again denied, than in the brain. Although, 
indeed, I am fully certain, that a group of lymphatic 
glands is no where found without lymphatic veffels, 1 
could not, however, affert that, where glands do not ap¬ 
pear, lymphatic veffels are wanting. By analogy drawn 
from the whole body, and all the vifcera, I am led to 
think that the brain is not deftitute of its aqueous veffels, 
and that they run in particular upon the furface, not in 
the middle of its fubftance, although I myfelf have never 
feen any other than thofe going on the choroid plexus to¬ 
wards the fenforium, and on the inner furface of the dura 
mater, in the courfe of the fuperior longitudinal linus. 
The induftrious and celebrated Sommering confirms the 
obfervations of King, Collins, and Pacchioni, who faw 
lymphatics upon the pia mater. I would alk, May they 
be joined with the glands of Pacchioni? May thefe cor- 
puicles fupply the place of lymphatic glands ?” 
It now remains for us to fpeak of the encephalon itfelf. 
Upon the furface of the brain lies the cortex, the fabric of 
which has been a long time controverted ; but it is now 
fufficiently evident, from anatomical injections, that the 
greater part of it confifts of mere veffels, which are every 
way inferted from the fmall branches Gf the pia mater, 
detached like little roots into the cortical fubftance. Thefe 
veffels, in their natural ftate, convey a juice much thinner 
than blood ; although, in fome difeafes, and by ftrangling, 
they often, efpecialiy in brutes and birds, receive even the 
