ANATOM Y. 
597 
be extremely elaftic, of an etherial or of an electrical na¬ 
ture ; the ancients fuppofed it to be incompreffible and 
watery, but of a lymphatic or albuminous nature. Indeed, 
it is not to be denied that we have many arguments again!! 
admitting either of thefe opinions. An eledtrical matter 
is doubtlels very powerful, and fit for motion; but then 
it is not confinable within the nerves, fince it penetrates 
throughout the whole animal to which it is communicated, 
exerting its force upon the flefn and fat, as well upon the 
nerves. In a living animal, the nerves only, or fuch parts 
as have nerves running through them, are affc&ed by ir¬ 
ritation ; and therefore this liquid inti ft be of a nature that 
will make it flow through, and be contained within, the 
narrow tubes of the nerves. And a ligature on the nerve 
takes away fenfe and motion, but cannot flop the motion 
of a torrent of eleCtrical matter. 
A watery and albuminous matter is common to mod of 
the juices in the human body, and may therefore be rea¬ 
dily granted to the juice of the nerves. The nervous fluid 
may probably be fimilar to the water exhaled into the ven¬ 
tricles of the brain ; and this opinion is ftrengthened by the 
flux of a gelatinous ,pr lymphatic juice from the brain of 
fillies, and nerves of large animals, when cut. But, are 
thefe properties Tuflicient to explain the wonderful force 
of convulfed nerves, obfervable in the diffeftions of living 
animals, even of the fmalleft infeits ? or to account for 
the great ftrength of mad and hyfterical people ? Is not 
this difficulty fomewhat leflened from the hvdroftatical ex¬ 
periments of attraction in fmall tubes; which, although 
it may explain the ftrength and motion, is nevertheiefs in- 
confiftent with the celerity? 
The nervous liquor, then, which is the inftrument of 
fenfe and motion, muft be exceedingly moveable, in order 
to carry the impreflions of fenfe, or the commands of the 
will, to the places of their deftination, without any re¬ 
markable delay'; nor can it receive its motions only from 
the heart. Moreover, it is very thin and invifible,'and 
deftitute of all tafte and fmell; yet reparable from the ali¬ 
ments. It is carefully to be diftinguilhed from that vili- 
ble vifcid liquor, exhaling from the velfeis in the inter¬ 
vals between the nervous cords. That this liquor moves 
through tubes rather than through a fpongy folid, we are 
perfuaded, from its “celerity, and from the analogy of all 
the fluids of the body, fat only excepted, running through 
their proper veffels. 
On the whole, therefore, it certainly appears, that, by 
the veflels of the cortex, fome kind of fluid is inftiiled 
into the tubes of the medulla; which fluid is continued 
through the extremely fmall tubes of the nerves to their 
extremities, and is the caufe both of fenfe and motion. 
But there will be a twofold motion in that humour ; the 
one flow and conflant, from the heart ; the other not con¬ 
tinual, but exceedingly fwift, which is excited either by 
fenfe, or any other caufe of motion ariling in the brain. 
The fame nerves moft evidently prefide over both fenfe 
and motion; as we cannot admit a diftinCtion between the 
two fyftems of motory and fenfitive nerves. If fenfe lome- 
times remain after motion is deftroyed, this feems to be be- 
caufe much more ftrength is required for the latter. Dying 
people hear and fee when they are incapable of motion. 
OF THE EYE. 
The eyes confift of hard and foft parts. The hard parts 
are the bones which form the orbits. The foft parts are 
of feveral kinds ; the moft etfential of which is the globe 
or ball of the eye ; the others are partly external and 
partly internal. The external parts are the fupercilia or 
eye-brows, the palpebrse or eye-lids, the caruncula la- 
chrymalis,- and the punfta lachrymalia ; and the internal 
parts are the mufcles, fat, lachrymal gland, nerves, and 
blood-veffels. The coats of the globe of the eye are of 
three kinds. Some of them form chiefly the fhell of the 
globe ; others are additional, being fixed only to a part of 
the globe; and others are capfular, which contain the 
humours. The coats which form the globe of the eye 
Vol. I. No. 38. 
are, the fckrotica or cornea , the choreide j, and the retina. 
The additional coats are two ; one called tcndinofa or albu¬ 
ginea, which forms the white of the eye; and fhe other 
conjunftiva. The capfular tunicas are likewife two, the 
vitrea and the cryftallina. The humours are three ; the 
aqueous, vitreous, and cryftalline. The firft may properly 
enough be called a humour , and is contained in a Space 
formed in the interfaces of tire anterior portion of the coats. 
The fecond, or vitreous humour, is contained in a parti¬ 
cular membranous capfula, and fills above three-fourths 
of the ftiell or cavity of the globe of the eye. It has been 
named vitreous, from its fuppofed refemblance to melted 
glafs ; but it is really more like the white of a new-laid 
egg. The cryftalline humour is fo called from its refem¬ 
blance to cryltal, and is often named (imply the c'yjialline. 
It is rather a gummy mafs than a humour, of a lenticular 
form, more convex on the back than on theTore-fide, and 
contained in a fine membrane called mamprana or capfula 
tryjlallina. 
The moft external, the thickeft, and ftrongeft, coat of 
the eye, is the fclerotica or cornea, and it invefts all the 
other parts of which the globe is compofed. It is divided 
into two portions, one called cornea opaca or fclerotica, the 
other cornea lucida, which is only a fmall fegment of the 
fphere (ituated anteriorly. The next coat of the globe ot 
the eye is the choroides, which is of a blackifh colour, 
more or lefs inclined to red, and adheres, by means of a 
great number of fmall veffels to the fclerotica, from the 
infertion of the optic nerve all the way to the cornea, where 
it leaves the circumference of the globe, and turns in¬ 
ward, to form a number of little proceffes termed ciliary, 
which are fituated at the edge of the cryftalline lens. At 
the interior edge of the choroides we find the iris compo- 
fed of two laminae ; the pofterior of which, being of the 
colour of a grape, was called uvea by the ancients. In the 
middle of the iris there is a hole termed pupil: this, in a 
foetus, is covered with a membrane called pupillaris , which 
generally difappears about the feventh month; or between 
the feventh and ninth month, according to Wrifberg. 
The laft coat proper to the eye is of a very different 
texture from that of the other two. It is white, foft, and 
tender, and, in a manner, medullary, or like a kind of pafte 
fpread upon a fine reticular web ; it lines the bottom of 
the eye, and is a continuation of the optic nerve. Some 
authors, as Zinn, affirm, that it terminates at the ciliary 
circle ; others, as Dr. Haller, reprefent the whole, or a 
part of it, as extended to the lens, and even as giving a 
covering to that humour; but Dr. Monro obferves, that 
it ends fome way behind the ciliary circle. 
The vitreous humour is a clear and very liquid gelati¬ 
nous fluid, contained in a fine tranfparent capfula, called 
tunica vitrea. It fills the greateft part of the globe of the 
eye, that is, almoft all that fpace which anfwers to the 
extent of the retina, except a fmall portion behind the 
uvea, where it forms a foffiila, in which the cryftalline lens 
is lodged. The cryftalline is a fmall lenticular body, of 
a firm confidence, and tranfparent like cryftal. It is con¬ 
tained in a tranfparent membranous capfula, and lodged in 
the anterior foftula of the vitreous humour. It is very im¬ 
properly called a humour, becaufe it may be handled and 
moulded into different fhapes by the fingers, and fometimes 
almoft diffolved by different reiterated comprellions, efpe- 
cially when taken out of the capfula. The aqueous hu¬ 
mour is a very limpid fluid, refembiing a kind of lympha 
or ferum, with a very fmall degree of vifeidity; but, in 
the foetus, and a fliort time after birth, it is of a reddifh 
colour. It fills the fpace between the cornea and iris, that 
between the iris and the cryftailine, and the hole of the 
papilla. Thefe two fpaces are called the chambers of the 
aqueous humour , and they are diftinguifhed into the anterior 
and pofterior. 
The tunica albuginea, commonly called tire white of the 
eye, and which appears on all the anterior convex fide of 
the globe, from the cornea to the beginning of the pofte¬ 
rior fide, is formed chiefly by the tendinous expanfion of 
7 N the 
