174. If a small portion of the inner coat of an artery be gently
scraped with a knife, or if the inside of the cheek be treated in a
similar manner, a little white soft substance is brought away on it,
called _epithelium_, a name given to it by Ruysch, from the delicate
layer of epidermis investing the female nipple, έπι, upon, θηλή,
_a nipple_. The epithelium of the human body is divided into three
kinds by microscopists--the _tesselated_, _pavement_, or _scaly_; the
_cylindrical_, or _conical_; and the _spheroidal_, or _glandular_. The
tesselated, as it exists in arteries, is represented in diagram No.
1, in three different stages--in the young person, in middle age, and
in the very old person; one stage gradually degenerating or changing
into the other, at each different period of life. It is composed of a
single layer of nucleated cells, of a flat, oval, round, hexagonal, or
polygonal form, and about 1/1400 of an inch in diameter, the nucleus
in each cell containing within itself one or more nucleoli, and even
several paler granules. The epithelium has a thickness proportioned
to the friction or pressure to which it is exposed, particularly when
covering the skin. In the arteries of the young, and in the mammalia
generally, the epithelium is strongly marked; in older persons, all
traces both of cells and nuclei have disappeared. It lines not only
the internal surface of the arteries and veins, but the mouth with its
mucous glands; the _conjunctiva_ of the eye; the pharynx and œsophagus;
the vagina and cervix uteri; the entrance of the female urethra, and
the serous membranes.
The _conical_ or cylindrical is composed of cells closely set together,
of a conical, cylindrical, or pyramidal form, about 1/1200 of an inch
long, each cell inclosing a flat nucleus, with nucleoli. It lines the
urethra in the female, from the entrance where the tesselated ends,
and extends inward to the urinary tubules of the kidneys; the greater
part of the male organs in a similar manner; the digestive canal and
gland-ducts, from the cardia to the anus.
The _spheroidal_ or _glandular_ epithelium consists of cells, more or
less circular or spherical in figure, each having a large nucleus in
its center. The epithelium is met with in all glandular organs, such as
the liver, kidney, lachrymal, and salivary glands, and in these cells
the proper secretion of the gland is developed. The tesselated and
cylindrical kinds are, on the contrary, more or less protective.
The two first kinds are sometimes ciliated, by the addition, at
their free extremities, of several fine, pellucid, blunt, and pliant
hairlike processes or cilia, about 1/5000 of an inch long, which are,
during life, in constant motion. This kind of epithelium, known as the
ciliary, lines the whole respiratory track of mucous membrane; the
_palpebral_ conjunctiva, as opposed to the tesselated on the eyeball;
the ventricles of the brain; the posterior half of the uterus, and the
Fallopian tubes.
The epithelium is placed upon the second layer of the internal coat,
which, from certain appearances of apertures or windows, has been
called the _perforated_ or _fenestrated_ layer. (See diagram No. 2.)
It can be peeled off in small pieces only, and shows under a power
of 250 diameters a series of well-marked fibers running in almost
parallel lines upon a comparatively structureless membrane, resembling
the inner layer of the cornea, as in the left-hand figure of the
diagram, the fibers being arranged in the length of the vessel. They
frequently bifurcate, and almost immediately join again, so that an
oval space, resembling a hole, is perceived. This is not always a hole
or perforation, as it is generally described to be, as may be seen
and proved by the fact that the supposed opening is sometimes filled
up by small bodies, like nuclei, as if the oval space were occupied
by a cell. This fenestrated layer varies in thickness in different
vessels, and is more strongly developed in the lower animals than in
man; by some authorities it is not regarded as a distinct layer, but
as the innermost layer of longitudinal fibers belonging to the middle
coat. When this layer is very thick, the fibers which are yellow do
not all run in the direction of the length of the vessel, for others
crossing at right angles may sometimes be observed, as delineated in
the right-hand figure of diagram No. 2. These two layers compose the
ancient inner coat of an artery, and are frequently the seat of disease.
[Illustration: No. 2.]
The middle coat, as it was termed, forms by much the greatest part
of the thickness of an artery, and, generally speaking, is of a more
or less yellow color. It appears fibrous to the naked eye, and can
be peeled off not unfrequently in a series of circular layers; when
examined microscopically, it is seen to be composed of _two_ sets of
fibers arranged in a circular direction. The inner layer is composed
principally of muscular fibers, of the organic or involuntary kind.
(See line marked 3 on the circular diagram.) The outer layer, marked
line 4 on the same diagram, is made up chiefly of elastic fibers, with
a much smaller amount of the muscular or contractile element. These
conjoined layers form the muscular coat of Mr. Hunter, the fibrous or
contractile coat of later anatomists, who denied its muscularity from
the supposed absence of fibrin--an error fallen into from chemical
science being unequal at that time to its discovery, or rather of its
more elementary part, called _protein_, the principal constituent
both of albumen and fibrin, which two are now found to differ from
each other in the addition only of three per cent, of sulphur. Mülder
says, in his “Animal and Vegetable Chemistry,” (Part II. p. 307:) “The
combinations of sulpho-phospho protein (_fibrin_ and _albumen_) and of
sulpho-protein _casein_ with acids, alkalies, and salts are especially
remarkable. Protein is soluble in weak alkalies. Since, therefore, the
serum of the blood is always slightly alkaline, being a proteate of
soda, with sulphur and phosphorus, it keeps the sulpho-phospho protein
in solution. This property is the cause of the blood remaining in a
liquid state--a chief requisite for animal life.
“If a weak alkaline solution of protein be neutralized by an acid,
the solubility of sulpho-phospho protein is greatly diminished. The
sulphuric and phosphoric acids, by not dissolving protein, stanch
bleeding. Acetic acid, by which protein is dissolved, does not, neither
does the hydrochloric.
“Protein, according to Mülder--although it is doubted by Liebig--is
a complex substance, consisting of several heterogeneous organic
compounds united into one whole, easily acted upon by strong reagents.
“If a protein compound be brought into contact with an alkali, ammonia
is immediately disengaged, and the alkaline solution can hardly be
made weak enough to prevent the disengagement of ammonia. If either
fibrin or coagulated albumen be dissolved in a weak potash lye, ammonia
is always perceptible. Protein, therefore, is always in a state of
decomposition, as serum is alkaline.”
In diagram No. 3, fig. 3, the organic or _involuntary_ muscular fibers
of the intestine are shown, consisting of more or less flattened
bands, the fibers of which are soft, and marked with minute granules,
sometimes exhibiting traces of nuclei. These purely muscular fibers are
most abundant next to the inner coat of the artery, and diminish in
number as they approach the outer layer, their place being occupied by
firmer and more elastic fibers of a yellow color, seen collectively in
the circular diagram, as line 4, and separately in diagram 3, fig. 4,
and in diagram 4.
[Illustration: No. 3.]
[Illustration: No. 4.]
The _involuntary_ muscular fibers of an artery do not always form
a continuous layer; they are often smaller than those found in the
intestines, bladder, and uterus, and occur as fusiform cells, detached
from each other, and having a large, club-shaped nucleus, as shown at
fig. 6 in diagram 3.
The _voluntary_ muscular fibers differ from the _involuntary_, in
having cylindrical fibers of much larger size, with transverse and
longitudinal markings, unlike the flattened fibers of less size of the
involuntary muscles, which have also a faintly granular appearance,
instead of the more determined transverse and longitudinal lines of the
voluntary muscles.
The _outer_ or _elastic_ layer of the ancient _middle_ coat,
represented by line 4 in the circular diagram, contains muscular
fibers, but it is formed principally of strong, elastic fibers
difficult of separation, and, when torn across, have curled
extremities, as shown in the diagram marked 4, differing only in size
from those found in the ligaments of the spine, and in the ligamentum
nuchæ of quadrupeds, as shown in the separate diagram marked 4.
The _external_ coat of an artery, divided also into _two_ layers, is
shown on the circular diagram by lines 5 and 6. These two layers are
composed of the yellow elastic fibers last noticed, and another set
of fibers, _white_ in color and _in_elastic in structure, arranged
in various directions; the _inner_ layer predominating in yellow
elastic, the outer layer in white inelastic fibers, constituting
a firm investment to all the other layers of which the artery is
composed. The white inelastic fibers are shown in diagram No. 3,
fig. 5, with a yellow elastic fiber curling round them. The constant
crossing and recrossing of these two sets of fibers form certain
spaces, which, when not in a compact form, become real spaces, meshes,
or areolæ, constituting what is now called areolar tissue, rather
than the cellular of the older anatomists, from the circumstance that
the areolæ communicate, and that perfect cells in any tissue do not.
These elements of areolar tissue can be readily distinguished by the
action of acetic acid, under which reagent the white fibers will
almost disappear, leaving only a slight trace of fibers containing
oval nuclei, as seen and marked in diagram 3, fig. 5. It is seen when
unraveled in _b_, diagram 5.
[Illustration: No. 5.
_a._ Yellow elastic fibers.
_b._ White inelastic fibers.
_c._ Nuclei.
_d._ Fiber, with nucleus.]
The inner layer of the middle coat, or muscular coat, as it may be
justly termed, forms, it will be seen, the greatest part of the
thickness of the wall of certain arteries, and in some instances, as in
the anterior tibial artery, constitutes nearly the entire thickness of
the vessel. The _internal_ coat in all is frequently seen puckered in a
longitudinal direction.
175. The arteries are supplied with blood by vessels of small size,
which do not come off immediately from the part of the artery they are
destined to supply, but principally from neighboring vessels. They are
called vasa vasorum. They are arranged precisely in the same manner as
those of the areolar tissue. A few of these vessels penetrate as far as
the middle or muscular coat, but do not reach the inner, which has no
vessels, proximity to the circulating fluid being apparently sufficient
for its nutrition.
Arteries are supplied with nervous influence by branches from the
sympathetic system running in their walls, and through their connection
by ganglions with the organs they supply with blood.
176. The cells, nuclei, and nucleoli alluded to are supposed to be
thus produced. In a shapeless, consistent, sometimes almost gelatinous
mass, to which the name of _cyto_-blastema or _formative substance_
has been given, containing the materials requisite for the production
of cells, small, round grains or nucleoli are perceived in the act of
formation. Around these grains a layer of granular matter is deposited,
which continually increases in thickness, and constitutes the kernel
or nucleus. This is oval shaped or round, almost always opaque, has
a granular surface, and is considered to be a vesicle, a little cell
itself. From the surface of this kernel a small, very thin transparent
vesicle is raised, appearing as a segment of a sphere, which soon
expands, and becomes so large, when full grown, that the kernel lies
as a minute corpuscle upon its interior wall; the material for its
formation being supplied by the cyto-blastema, it is converted into a
vesicle by the kernel which is first formed, its embryo existing in the
formative substance.
The first trace of organization is the production of a small,
perceptible body, or nucleolus, which deposits on the surface a
granular substance from the cyto-blastema, to give rise to a little
producing organ, the kernel or nucleus. This further transforms the
surrounding cyto-blastema into a granular surface, from which the
vesicle is formed, raised, expanded, and filled with a liquid, in which
vesicle thus enlarged the kernel remains inclosed and adhering to a
certain spot of its wall.
If two nucleoli lie close to one another, they coalesce and become one
solid mass, capable of producing one cell only, containing one kernel
and two nucleoli. This view is that of Schleiden and Schwann, supported
by Mülder, but not entirely approved by Henle; inasmuch as no kernel
can be perceived in the cells of many cellular systems while in the
act of formation. In the elementary parts of animals which have long
since lost their cellular form, the remnants of kernels are frequently
found, as has been demonstrated in the preceding diagrams. The manner,
however, in which the elementary first-seen granules are formed in the
cyto-blastema, science has not yet been able to discover. The chemists
have proved that all elementary organic substances consist of carbon,
hydrogen, oxygen, and nitrogen, susceptible of endless modifications
of their respective forces, under which an organic molecule or ovum is
produced, and after that, under certain circumstances, an animal such
as man.
177. When the current of blood through the main trunk of the arteries
of an extremity is cut off, the circulation is carried on by the
collateral branches. This collateral circulation is more perfect, more
active in young persons during the increase or growth of the body,
than it is either at maturity or in the decline of life. The important
point is not, however, alone referable to the time of life at which the
continuity and permeability of the main trunk cease to exist, but to
the nature of the disease or injury which has given rise to it.
When an aneurismal limb has been injected, on which an operation has
not been performed, the collateral vessels have all been found larger
and more fully shown than on the opposite side, although not to the
same extent as in cases of a similar nature in which the operation has
been done.
It is necessary that this enlargement of the collateral branches should
lake place at an early period, because in many cases of aneurism
the artery beyond or below the tumor is obliterated long before any
operation is performed. The main supply of blood has been already cut
off from the extremity, and the operation adds very little to the
derangement of the circulation which has for some time taken place
below the tumor.
When an operation has been successfully performed for aneurism, and
the patient has died some time afterward, dissection has shown various
arteries enlarged, both above and below the part where the trunk was
obliterated by the ligature; and not only an enlargement of arteries,
which, from their regularity have received names, but others have
been developed not usually known to exist, or not of a size to be
conveniently traced. These through their frequent anastomoses bring the
blood at last into several large trunks, by which it is again conveyed
to the original vessel below all and every obstruction which may have
taken place; thus compensating by a circuitous route for the loss of
the direct supply. The principal object of inquiry is, do these vessels
always exist, or at what period of time do they begin to enlarge, so as
to enable them to carry on the circulation, in the manner in which it
is presumed to be done?--for few will assert that the enlargement of
these particular collateral vessels was an accidental play of nature,
and existed previously to the commencement of the disease or injury
for which the operation was performed. On this point, the theory of
the operation for aneurism and its applicability to wounded arteries
appears to hinge; and, what is of more importance, on which the
practice resulting from it depends.
Two distinct kinds of collateral circulation are at present
acknowledged: one by direct large communicating arteries; the other
through the direct medium of the capillary vessels inosculating
with each other. Where direct communicating arteries exist, little
subsequent change beyond enlargement takes place in them. It is
otherwise with the indirect capillary vessels. When the radial or
ulnar artery has been divided in the hand, the blood will not only
flow readily from each end of the divided vessel, but equally red and
arterial from both, the communication being through direct arterial
branches from one vessel to the other. It will also be red and arterial
if the division take place at the wrist, and may be so in the brachial;
but if the femoral in the lower part of the thigh be wounded, the color
of the blood issuing from the lower end of the artery, if any issue at
all, will be _dark or venous_. It is so, because it has been obtained
from the capillary arteries, which in this case being empty received
blood by regurgitation from the veins, the valves of which when present
do not prevent its reflux course. If a limb be injected and carefully
dissected four or five days after a ligature has been placed during
life high up on the principal trunk, the capillary vessels will be
seen to be well injected; but few or none will be found large enough
to admit of their inosculation being traced throughout. If another
limb be injected and dissected, some sixty days after the ligature
has been applied, a difference will be distinctly observed between
the two preparations. In the latter, the capillaries will not appear
to be so fully injected, but several larger and more tortuous vessels
will be found in situations where they were not expected to exist;
and the anastomoses of these one with another, generally by arches,
may be traced to their communication with the principal trunk, both
above and below the obliterated part. If an incision were made in the
nearest pervious portion of the lower part of an artery in the thigh of
a person who had undergone this operation, arterial blood would issue
from it. The communication would have become direct by communicating
branches, and the capillaries would have returned to their accustomed
duties.
178. During the first twenty-four hours after the division of an artery
such as the femoral, or the application of a ligature, the temperature
of the limb is commonly diminished; after that period, and as the
action of increase takes place, the temperature is usually from three
to five degrees higher than in the opposite healthy limb. At the end of
from eighteen to twenty-eight days, in a successful case, it is found
to be equal in both.
It is asserted by some sanguine supporters of the all-powerful
influence of the collateral circulation, that it is sufficient at all
times, and under all natural circumstances, to maintain the life of
the extremity. The practice of the Peninsular war proved the fallacy
of this opinion in too many instances to admit of any doubt of its
inadequacy to do so in the lower extremity after the division of the
femoral artery, under ordinary circumstances. The fact of enlargement
or of a new development of vessels having taken place after the
commencement of disease or the reception of an injury, has been
demonstrated by dissection, and it is through them the life of the limb
is to be preserved; but time is required for their development. When a
limb is lost through mortification, as the consequence of a division
or obstruction of the principal artery, it usually takes place after
the infliction of a sudden injury, in consequence of these collateral
branches not having had time to enlarge.
179. The collateral circulation is therefore not the same, and is not
in the same stage of preparation, in a limb suffering from a divided
or wounded artery, as in one in which an aneurism has for some time
existed; this is the reason why mortification is more common after
wounded arteries than after operations for aneurism.