ANATOMY. 
400 
sible, and that this must depend upon the 
Uerves (which, all conjoined, would not make 
a cord of an inch diameter) being divided 
into branches or filaments, to be dispersed 
through all these minute parts, we must be 
convinced that the nervous fibrils are very 
small. Prom the examination of the mini- 
mum visible, it is demonstrated, that each 
fibre in the retina of the eye, or expanded 
optic nerve, cannot exceed the size of the 
32400th part of a hair. 
The medullary substance of which the ner- 
vous fibrils are composed, is very tender, and 
would not be able to resist such forces as the 
nerves are exposed to within the bones, nor 
even the common force of the circulating 
fluids, were not the pia mater and tunica 
arachnoides continued upon them; the for- 
mer giving them firmness and strength, and 
the latter furnishing a cellular coat to connect 
the threads of the nerves, to let them lie soft 
and moist, and to support the vessels which 
go with them. It is this cellular substance 
that is distended, when air is forced through a 
blow-pipe thrust into a nerve ; and that makes 
a nerve appear all spongy, after being dis- 
tended with air till it dries : the proper ner- 
vous fibrils shrivelling so in drying, that they 
scarce can be observed. These coats would 
not make the nerves strong enough to bear 
the stretching and pressure they are exposed 
to in their course to the different parts of the 
body; and therefore, where the nerves go 
out at the holes in the cranium and spine, 
the dura mater is generally wrapped closely 
round them to collect their disgregated fibres 
into tight firm cords; and that the tension 
which they may happen to be exposed to • 
may not injure them before they have got 
this additional coat, it is firmly fixed to the 
sides of the holes in the bones through which 
they pass. 
The nervous cords, thus composed of ner- 
vous fibrils, cellular coat, pia and dura ma- 
ter, have such numerous blood-vessels, that 
after their arteries only are injected, the 
whole cord is tinged of the colour of the in- 
jected liquor ; and if the injection is pushed 
violently, the cellular substance of the nerves 
is at last distended with it. 
A nervous cord, such as has been just now 
described, has very little elasticity, compared 
with several other parts of the body. When 
cut out of the body, it does not become ob- 
servably shorter, while the blood-vessels con- 
tract three-eighths of their length. 
Nerves are generally lodged in a cellular 
or fatty substance, and have their course in 
the interstices of muscles and other active 
organs, where they are guarded from pres- 
sure ; but in several parts they are so placed, 
as if it was intended that they should there 
suffer the vibrating force of arteries, or the 
pressure of the contracting fibres of muscles. 
The larger cords of the nerves divide into 
branches as they go off to the different parts ; 
the branches being smaller than the trunk 
from which they come, though not propor- 
tionally so ; for, as Soemmerring observes, 
they rather form cones with apices toward 
the brain, and bases toward the superficies of 
the body. Where the nerves separate, they 
generally make an acute angle. 
In several places different nerves unite 
into one cord, which is commonly larger than 
any of the nerves which form it. Several 
nerves, particularly those which are distri- 
buted to the bowels, after such union, sud- 
denly form a hard knot, considerably larger 
than all the nerves of which it is made. '1 hese 
knots were called corpora olivaria, and are now 
generally named ganglions. The ganglions 
have thicker coats, more numerous and larger 
blood-vessels, than the nerves ; so that they 
appear more red and muscular. On dissect- 
ing the ganglions, fibres are seen running 
longitudinally in their axes, and other fibres 
are derived from their sides in an oblique di- 
rection to the longitudinal ones. Commonly 
numerous small nerves, which conjunctly are 
not equal to the size of the ganglion, are sent 
out from it, but with a structure no way dif- 
ferent from that of other nerves. 
The nerves sent to the organs of the semen 
lose there their firm coats, and terminate in 
a pulpy substance. The optic nerves are 
expanded into the soft tender webs, the re- 
tins. The auditory nerve has scarce the con- 
sistence of mucus in the vestibulum, cochlea, 
and semi-circular canals of each ear. The 
papillae of the nose, tongue, and skin, are 
very soft. The nerves of muscles can likewise 
be traced till they seem to lose their coats by 
becoming very soft ; from which, and what 
we observed of the sensatory nerves, there is 
reason to conclude, that the muscular nerves 
are also pulpy at their terminations, which 
we cannot indeed prosecute by dissection. 
It would seem necessary that the extremities 
of the nerves should continue in this soft flex- 
ible state in order to perform their functions 
right ; for, in proportion as parts become rigid 
and firm by age, or any other cause, they 
lose their sensibility, and their motions are 
more difficultly performed. 
Though the fibres in a nervous coat are 
firmly connected, and frequently different 
nerves join into one trunk, or into the same 
ganglion, yet the sensation of each part of the 
body is so very distinct, and we have so much 
the power of moving the muscles separately, 
that, if the nerves are principal agents in these 
two functions, we have reason to believe that 
there is no union, confusion, or immediate 
communication of the proper nervous fibrils, 
but that each fibre remains distinct from the 
origin to its termination. 
Many experiments and observations con- 
cur in proving, that when nerves are com- 
pressed, cut, or any other way destroyed/the 
parts served by such nerves, farther from the 
head dr spine than where the injuring cause 
has been applied, have their sensations, mo- 
tions, and nourishment, weakened or lost, 
while no such effects are seen in the parts 
nearer to the origin of those nerves ; and in 
such experiments, where the cause impeding 
the nerves from exerting themselves could be 
removed, and the structure of the nerves not 
injured, (as, for example, when a ligature 
made upon a nerve, and stopping its influ- 
ence, has been taken away,) the motion and 
sensation of the parts soon were restored ; 
from which it would appear, that the nerves 
are principal instillments in our sensations 
and motions ; and that this influence of the 
nerves is not inherent in them, unless the 
communication between these cords and their 
origin is preserved. 
Experiments and observations show, too, 
that when parts of the encephalon or spinal 
marrow have been irritated, compressed, or 
destroyed, the parts of the body, whose 
nerves had their origin from such affected 
parts of the encephalon or spinal marrow, 
became convulsed, paralytic, insensible, or 
wasted ; and in such cases, where the injuring 
cause could be removed from the origin of 
the nerves, the morbid symptoms observed 
in the parts to which these nerves were dis- 
tributed, went off upon the removal of that 
cause; from which it is thought reasonable- ter 
conclude, that the nerves must not only ha\ e 
a communication with their origin, but that 
the influence they have upon the parts they! 
are distributed to, depends upon the influence! 
which they derive from the medulla encephali j 
and spinalis. 
If such causes produce constantly such ef-l 
fects in us and other creatures living in nearly! 
the same circumstances as we do, the con-j 
elusions already made must be good, notwith-J 
standing the examples of children and otheil 
creatures being born without brains or spinal! 
marrow, or notwithstanding that the brains of 
adult creatures can be much changed in their 
texture by diseases, and that tortoises and 
some other animals continue to move a coni 
siderable time after their heads are cut off! 
We may be ignorant of the particular cir- 
cumstances requisite or necessary to the! 
being or well being of this or that particular 
creature, and we may be unable to account 
for a great many phenomena ; but we must 
believe our eyes in the examination of facts ; 
and if we see constantly such consequences 
from such actions, we cannot but conclude 
the one to be the cause and the other the 
effect. It would be as unjust to deny the 
conclusions just now made, because of the 
seemingly preternatural phenomena mention] 
edat the beginning of this head, as it would be 
to deny the necessity of the circulation of the 
blood in us and most quadrupeds, because a 
frog can jump, or a tortoise can walk aboui 
after all the bowels of its thorax and abdo- 
men are taken out, or because the different 
parts of a worm crawl after it lias been cuf 
into a great many pieces. It is therefore at 
most universally allowed, that the nerves are 
principal instruments in our sensations, mo- 
tions, and nourishment ; and that the influence 
which they have, is communicated from theif 
origin, the encephalon and medulla spinalis. 
But authors are far from agreeing about the 
manner in which this influence is communi- 
cated, or in what way nerves act to produce 
their effects : some alleging that the nen 
vous fibres are all solid cords, acting by das 
ticity, or vibration ; others maintaining that 
these fibres are small pipes conveying liquors 
by means of which their effects are produced 
and certainly very considerable objection! 
may be made to both doctrines. 
