53 2 A N A 1 
of thofe of the medulla, is diftitiftly fcen in the feventh, 
fourth, and fifth, pairs of nerves'; 
The nerves, then, are compofed of many fibrous threads, 
lying parallel, or nearly fo, to each other, as they (hoot off 
from the medulla. At the origin of moft of the nerves 
within the fkull, this fibrous texture is perceptible; and, 
in the cauda equina of the fpinal. marrow, they may be 
divided into threads fo exceedingly minute, that they are 
fcarcely vifible to the naked eye; yet even thefe fibres, 
when examined with a microfcope, appear to be formed 
of a considerable number of fibrils, much more minute. 
It is perhaps not poffible to eftimate the fize of the imalleft 
of thefe fibres. Were the nerves, which are divided over 
the whole body, conjoined into a cord, the diameter would 
not be an inch ; yet even the moll minute part of the body 
is fenfible: and this mud: depend on each particular point 
being fupplied with nerve. We muff therefore be con¬ 
vinced, that the nervous fibrils are aftonifhingly fmall. 
Nay, it has been demonftrated, from the extreme minute 
portions of matter which are vifible to the fight, that each 
fibre in the retina of the eye cannot exceed in diameter 
the thirty-two thoufand four hundredth part of a hair. 
The medullary matter, of which the nervous fibrils are 
formed, is exceedingly delicate and foft. They are there¬ 
fore connected together by cellular fubftance, and protect¬ 
ed from injury by coats formed of the dura and pia mater. 
Thefe nervous cords are fo liberally fupplied with blood- 
veffels, that, when their arteries only are injeCted, the 
whole cord appears to be tinged with the colour of the 
injeCted liquor. 
The nerves, after being given out by the brain or fpinal 
marrow, are generally, like the blood-veffels, divided into 
branches : but they pafs off, or feparate, from each other, 
in acute angles, and often in a retrograde direction. The 
fibrils of the nerves, however, are not fplit at thefe divi- 
fions fo as to form fmaller threads ; but the original fibres, 
continued from the brain itfelf, recede from each other 
by an opening of the cellular fubftance by which they are 
united : fo that, after the nervous fibril has left the brain, 
it appears, in faCt, to undergo no change till its ultimate 
termination. The nervous cords generally appear to grow 
fofter during their courfe, till at length, at their termina¬ 
tion, they feem to be loft in a mucus or pulp. Hence it 
lias been fuppofed, that during their progrefs they depo- 
fit the coats which they had received from the pia and 
dura mater ; and that, to be capable of fenfation, or the 
other purpofes which the nerve is to effeCt in the fyftem, 
it is neceftary that the medullary part fliould be laid per¬ 
fectly bare at its termination. It is however alleged, that 
every filament of a nerve retains, at its termination, a co¬ 
vering of the pia mater, by the veffels of which it is nou- 
rifhed and preferved in that ftate, upon which the faculty 
of-receiving impreflions, and the power of aCtion, depend. 
In fome inftances we obferve different nerves uniting fo 
as to form one cord ; this, in fome meafure, refembles the 
inofculations which take place in the fanguiferous fyftem. 
After fuch an union, we frequently find, particularly in 
thofe which are diftributed to the bowels, a hard knot is 
formed, eonftderably larger than the nerves which are 
thus united into a cord. Thefo knots are generally termed 
ganglions: t-hey have thicker coats, and are more liberally 
fupplied with blood-veffels, than the nerves. Dr. Monro 
obferves, that they are full of nervous fibrilli, intermixed 
with a yellowifh or reddifti brown fubftance, fomewhat fi- 
milar to the cortical fubftance of the brain. Hence he fup- 
pofes, that they are fources of nervous matter or energy. 
Derived from this origin, (the brain,) we find forty pairs 
of nerves diftributed through the body, viz. ten which 
immediately proceed from the encephalon, and thirty 
which pafs off from the medulla fpinalis. Of thefe we 
find four proceeding immediately from the brain, which 
are deftined to receive various impreflions from external 
objects, by which the fenfations of fight, hearing, fmell, 
and tafte, are produced. The reft are diftributed to the 
various mufcles/and extended'on the furfaceof the body, 
O M Y. 
effecting the motion of the mufcular folid, and producing^ 
in every part of the fyftem, by impreftions from different 
fubftances, that fenfation which we call foeling. To what¬ 
ever purpofe the nerve is applied, whether to receive fen¬ 
fation, or to excite motion, we do not perceive any differ¬ 
ence in its ftruclure ; it is in every inftance formed of the 
fame foft medullary fibre. We perceive, indeed, that the 
optic nerve, at its termination, is extended into a delicate 
web, which covers the lurface' of the eye. The portio 
mollis, alfo, of the auditory nerve, is extended in a very 
foft pulpy form on the inner camera of the ear ; but, ex¬ 
cepting thefe two, we obferve no variations in the external 
appearance of the nc-r,ves at their terminations : nor do w e 
difcover, even in thefe, any peculiarities of ftruclure, by 
which we can be able to allign a reafon why the one fliould 
receive the rays of light, reflected from furrounding bo¬ 
dies, by which the fenfation of light is produced ; 01^why 
the other fliould be affefted by percuflions of the air, fo 
as to communicate the different fenfations of found. We 
obferve, indeed, that feveral of the nerves which are de- 
figned to communicate particular fenfations to the mind, 
are furnifhed with peculiar apparatus. Such we obferve 
in the wonderful ftnnfture of the eye ; and in the ftriking 
contrivance exhibited in the veftibule, cochlea, and fenu- 
circular canals of the ear. That thefe apparatus are effen- 
tially requiiite to enable the nervous matter to receive par¬ 
ticular fenfations, cannot be doubted; but it muff remain 
as a queftion which probably will never be determined, 
whether the medullary matter itfelf does not undergo fome 
modification, by which it is fitted refpebtively toreceive 
impreflions from light, found, odorous or fapid fubftan- 
ces : to difiinguifli the nature of other fubftances by what 
we call feeling ; or to excite the mufcular aftion. What¬ 
ever may be in this, we are certain, that fenfation alto¬ 
gether, artd mufcular motion in a great meafure, are pro¬ 
duced by the operation of various agents upon the nervous 
fyftem. If a nerve is irritated, acute pain is induced ; and, 
in proportion to the number of nerves with which any 
part of the body fupplied, it is poffeffcd of a greater or 
fmaller fenfibility. Thus, in the eye and the penis, which 
are plentifully fupplied with nerves, the feeling is extreme¬ 
ly acute ; and the‘tendons, ligaments, bones, and cartila¬ 
ges, into which few nerves enter, have in the found ftate 
little or no fenfation. But if, bn the other hand, the 
communication between the fentient extremities of the 
nerves and the fenforium commune is obftrudled, as by 
the divifion of the nerve, the fenfation of the part beyond 
the divifion is entirely deftroyed. 
Having thus taken a curfory vi,ew of the outline of ana¬ 
tomy, with the phyliology appertaining thereto, we fliall 
now proceed to the dilfebtion of the human fubjebl; noting 
by the way the moft recent and important difcoveries, cal¬ 
culated to improve the knowledge and fuccefs of medical 
prablice. 
Of the BONES, or HU M A N S K E L ET 0 N. 
However folid and compact adult bones are, yet thqy 
were once cartilages, membranes, nay, a mere jelly. This 
needs no farther proof than repeated obfervations of em¬ 
bryos when diffeded : and how much more tender muff 
the bones be before that time, when neither knife nor eye 
is capable to difcover the lead rudiments of them? By 
degrees they become more folid, then affume the nature 
of griftles, and at laft oflify; the cohelion of their plates 
and fibres always inCreafing in proportion to their increafed 
folidities; as is evident from the time neceftary to unravel 
the texture of bones of people of different ages, or of denfe 
and of fpongy bones, or of the different parts of the fame 
bone, and from the more tedious exfoliations of the bones 
of adults than of children. As the folidity of bones in- 
creafes, their periofteum more eafily feparates from them. 
When bones are membranous, the periofteum cannot be 
diftinguiflied ; they appear to be the fame fubftance. When 
they are cartilage's, their membrane adheres fo firmly to 
them, that it is difficult to feparate it from them. Where 
