vaso-constrictor kind, and the dilatation which has been mentioned has
been due to cessation of the constrictor energy normally passing
outward. This may be termed a passive dilatation. It is the kind
produced by division of any one of the sympathetic ganglia or cords.
But further experiments have shown that another kind of dilatation may
be produced, traceable not to a mere cessation of constrictor
impulses, but to an impulse of a positive kind sent to the local
ganglia and resulting in a sudden suspension of their activity. Such
an impulse is really an inhibitory impulse arresting the action of the
ganglia in spite of the continued stimulus sent to them from the
central nervous system. Its result is a dilatation of the arteries,
produced by the blood-pressure within them, which may be termed an
active dilatation. Thus, Bernard {1245} found that irritation of the
chorda tympani caused an immediate flow of blood to the submaxillary
gland because of the dilatation of its vessels. And Dastre and
Morat[2] have demonstrated a similar effect in the head and
extremities after irritation of portions of the cervical sympathetic
and of the peripheral nerves.
[Footnote 2: See _Archives de Physiologie_, “Vaso-dilateurs,” 1879,
1880, 1882; _Comptes rendus de l'Academie des Sciences_, 1880, pp. 393
and 441.]
Much confusion has arisen from the use of the term active dilatation,
and many explanations of its mechanism have been offered. At first it
was supposed that a system of longitudinal fibres in the vessel-wall
acted as opponents to the circular constrictor fibres. This theory,
originating with Stilling and Duchenne,[3] has been lately revived by
Anrep and Cybulski.[4] They hold that since a vessel elongates as well
as dilates with every heart-beat, its total distension is the result
of two factors—viz. transverse and horizontal distension. If one of
these is neutralized, they claim that the other will be increased.
Longitudinal fibres in the wall by preventing elongation may thus
allow the entire force of the heart to be expended in dilating the
vessel. This theory has not, however, been accepted, and with that of
Schiff, that contractile elements of the connective tissue surrounding
the vessel-walls could pull outward the walls and thus dilate the
vessel, has lapsed, because of lack of demonstration of the necessary
anatomical structure in all arterioles. Another theoretical
explanation, that dilatation of the arteries is caused by contraction
of the veins damming back the blood, is disproved by the fact proven
by Dastre and Morat, that blood-pressure increases instead of
diminishing in the veins during vaso-dilator action. Legros[5] and
Onimus,[6] noticing the normal occurrence of a peristaltic motion of
centrifugal direction in the arteries of the retina, which if
increased produced a certain degree of hyperæmia, attempted to explain
the phenomena of dilatation by supposing a sudden increase of
peristalsis. But Vulpian has proved that the peristalsis is both too
slight and too slow in its effects to account for the rapid action of
the vaso-dilators, and Dastre and Morat have shown that the
peristalsis, not being synchronous with the heart-beat, really impedes
the flow of blood. The last theory to be mentioned has a chemical
basis, and is known as the theory of attraction (Brown-Séquard,
Severini.) According to this, the organs, when active, manifest
increased metabolism, to maintain which the blood is drawn toward them
by the chemical changes in progress. This theory was based on the fact
that irritation of the chorda tympani causes an increased secretion of
saliva as well as a congestion of the submaxillary gland. It is now
known, however, that these two acts are independent of each other, as
either can be suspended while the other continues. Hence this theory
too has lapsed.
[Footnote 3: Von Recklinghausen, _Handbuch der Pathologie des
Kreislaufs und der Ernahrung_, 1883, where a full bibliography is to
be found.]
[Footnote 4: _St. Petersburg Med. Wochenschrift_, 1884, i. 215.]
[Footnote 5: _Des Nerfs vaso-moteurs_, Thèse de Concours, Paris,
1873.]
[Footnote 6: _Des Congestions actives_, Paris, 1874.]
The view already stated, that vaso-dilators act by inhibiting local
ganglia which cause contraction, is now accepted, especially since it
is found that such inhibitory activity is constantly displayed by
other parts of the nervous system, and is competent to explain the
facts. The active dilatation produced by the inhibition of the action
of the local ganglia is {1246} therefore to be clearly distinguished
from the passive dilatation caused by the cessation of normal tonic
impulses sent to them from higher centres. The former is a positive
active vaso-dilator phenomenon. The latter is a negative paralytic
vaso-constrictor phenomenon. The former is more effective, the
dilatation being greater in degree and more permanent than the latter,
and resembles exactly the dilatation produced experimentally by
exhaustion of the activity of the local ganglia by over-stimulation of
the constrictors.[7]
[Footnote 7: Goltz, _Arch. f. d. gesammt. Physiol._, xi. 92.]
An important point of contrast which has been established between
vaso-constrictor and vaso-dilator impulses is that while the former
are constant the latter are intermittent. Hence they cannot be
regarded as opponents of one another. In a normal quiescent state
vaso-constrictor energy is always being supplied to counteract the
continued intravascular pressure ever renewed with the cardiac
systole. The vaso-dilators are inactive. In an organ thrown into
functional activity an increased flow of blood at once takes place,
proportionate to the work being done by the organ. Such a functional
hyperæmia might be produced either by a cessation of constrictor
impulses or by an inhibition of their effects. It is by the latter
means and through the vaso-dilators that it is produced, and it is
probably the chief function of the vaso-dilator nerves to regulate the
blood-supply in accordance with the need of a part. For this reason
these nerves have been supposed to pass with the motor nerves to the
muscles. As few organs exist without a possible use, it is probable
that vaso-dilators pass to all parts of the body, as Vulpian asserts,
though they have not been demonstrated in every organ or every part.
Like the vaso-constrictors, the vaso-dilators can be traced to the
spinal cord, and their centres there are governed by a general centre
in the medulla, which in turn may be affected by impulses from the
cortex. A destructive lesion in any part of the vaso-dilator system
does not produce as marked effects as one involving the
vaso-constrictor system, since the symptoms of such a lesion will only
appear when the vaso-dilators are called into play. When the
vaso-dilator nerve to the submaxillary gland is cut, no change is
observed until by some sapid substance put in the mouth its function
should be excited, when the gland is no longer found to flush with
blood as in the normal state. It is by means of the vaso-dilators that
erectile organs become engorged with blood. Eckhard has shown the
nervi erigentes of the penis to be vaso-dilator nerves. If they are
destroyed, the organs will not respond to the wonted stimulus—a
symptom which, however, would only be noticed at intervals. An
irritative lesion of the vaso-dilator system may produce permanent
congestion of an organ or part, but this seems to be rather more rare
than a congestion from paralysis of the constrictors. It is seen in
injuries of the peripheral nerves.
Origin of the Vaso-motor Nerves.—The exact course of the
vaso-constrictor and vaso-dilator nerves has been traced from various
parts into the central nervous system by the careful experiments of
Dastre and Morat,[8] and more recently by Gaskell.[9] It is now
established that they {1247} exist as separate nerves, although they
often run together, and that they usually enter the spinal cord at
different levels.
[Footnote 8: See _Comptes rendus Soc. de Biologie_, and _Arch. de
Physiol._, 1879-84.]
[Footnote 9: W. H. Gaskell, “On the Structure and Function of the
Nerves which Innervate the Vascular System,” _Journ. of Physiol._,
Jan., 1886.]
If the various spinal nerves be cut singly from above downward, and
the effects noted, and if the peripheral ends be irritated and the
effects noted, and if with proper care the results be analyzed and
compared, it will be found that the anatomical connections of the
segments of the spinal cord with the sympathetic ganglia, which are so
evident at each level, are not the ones by which physiological
impulses pass out at that level. The vaso-constrictors of the head,
which can be traced to the superior cervical ganglion, do not come
from the upper cervical region of the cord, as might be supposed from
the connections of that ganglion, but from the first three dorsal
nerves. They reach the ganglion through the sympathetic cord in the
neck, having traversed the inferior cervical ganglion on the way.
There are vaso-constrictors in the cranial cavity which accompany the
third, fifth, seventh, and twelfth nerves to the eye, face, and mouth.
It is still undecided whether they originate in the cord and medulla,
issue in the spinal accessory nerve, and with it enter the cranium
(Gaskell), or reach those nerves by way of the carotid and vertebral
plexus from the inferior cervical ganglion (Dastre and Morat). The
vaso-constrictors of the arm, which can be traced to the inferior
cervical and upper thoracic ganglia and to the thoracic sympathetic
cord, are derived from the seven upper dorsal nerves. It is true that
division of the roots of the brachial plexus causes a slight
dilatation of the vessels of the arm, but this is so much increased
when the dorsal nerve-roots are divided that it is evident that it is
through them that the constrictor fibres chiefly pass. The
vaso-constrictors of the leg, which can be traced into the second and
third lumbar ganglia and lumbar sympathetic cord, are derived from the
five lower dorsal and first lumbar nerves, and only join the crural
and sciatic nerves after passing through the abdominal sympathetic.
The thoracic viscera are probably supplied partly through the branches
of the inferior cervical and thoracic ganglia, and partly through the
pneumogastric, the latter statement being disputed by Gaskell. The
abdominal viscera are supplied partly through the splanchnic nerves,
which are made up of fibres issuing from the cord in the fifth to the
twelfth dorsal nerves inclusive, and partly through the pneumogastric.
Thus the dorsal region of the cord is the origin of the majority of
vaso-constrictor fibres in the body.
The origin of the vaso-dilators is also to be traced to the dorsal
cord. The vaso-dilators of the bucco-facial region come from the
second to the fifth dorsal nerves, whence they pass to the first
thoracic ganglion, and thence by the annulus of Vieussens into the
cervical sympathetic cord. Those of the eye, head, and ear come from
the same ganglion, but have their spinal origin in the eighth cervical
and first dorsal nerves. Those of the arm are traced into the upper
thoracic sympathetic cord, which they reach from the five upper dorsal
and last cervical nerves. Those of the leg are traced to the first and
second lumbar ganglia and the lumbar sympathetic cord, which they
reach from all the dorsal nerves from the sixth downward. Gaskell
holds, however, that the vaso-dilators of the extremities pass out of
the cord in the cervical and lumbar plexuses and accompany the
cerebro-spinal nerves. The vaso-dilators of the thorax and {1248}
abdomen are supposed to pass in the pulmonary plexus and splanchnic
nerves, but this is not yet fully determined.
Gaskell[10] believes that vaso-motor nerves can be distinguished from
motor and sensory nerves in the spinal nerve-roots by the smallness of
their calibre. He finds such fine fibres only in the spinal
nerve-roots between the second dorsal and second lumbar segments of
the cord (in the dog), and in the three upper roots of the spinal
accessory nerve. According to his account, the vaso-constrictors issue
from the spinal cord in both anterior and posterior nerve-roots as
medullated fibres, and pass to the sympathetic ganglia lying on the
vertebræ (proximal or lateral ganglia); there they lose their
medullary sheath, and either end in cells whence new fibres issue, or
more probably pass directly onward as non-medullated fibres, having a
connection with the unipolar cells of the ganglia only for purposes of
nutrition. The number of fibres issuing from any one ganglion is much
greater than the number entering it from the cord; hence it is
supposed that each medullated fibre splits up into a group of
non-medullated fibres; which is possible, as the researches of Ranvier
have shown that each axis-cylinder is made up of numerous fibrils.
Leaving these ganglia, the nerves pass either to the second series of
ganglia (distal or collateral ganglia), whence they issue in plexuses
to enter the vessels, or to the vessels directly, where they divide in
plexuses. It is in the meshes of the plexus that the local ganglia of
the vessel-walls are found. The vaso-dilators are thought to differ
from the vaso-constrictors in passing directly to the distal ganglia
as medullated fibres, not being connected in any way with the proximal
ganglia. Gaskell makes no statement regarding their limits of origin
from the cord, except to state that the nervi erigentes issue with the
sacral nerves. He agrees with the view that the vaso-dilators act as
inhibitory nerves upon the local ganglia.
[Footnote 10: _Journal of Physiology_, Jan., 1886.]
Vaso-motor Reflexes.—Thus far, reference has been made only to fibres
whose direction of transmission is centrifugal, and whose exit from
the spinal cord is by the anterior nerve-roots. There are other
fibres, however, through which centripetal impulses pass, and these
enter the spinal cord with the posterior nerve-roots. The function of
these fibres is to transmit sensory impulses inward to reflex centres,
and thus set in action motor mechanisms of a vaso-constrictor or
dilator kind whose effects are produced at the periphery. There are,
therefore, vaso-motor reflexes, as well as skin and tendon reflexes,
whose centres are in the spinal cord. These reflex acts may be excited
by impulses reaching the centre not only through the vaso-motor
centripetal nerves, but also through the sensory nerves of the
cerebro-spinal system. The effect of changes of temperature on the
circulation in the skin (if the right hand be plunged in cold water
there is a fall of temperature in the left hand), the effect of pain
upon the color of the face and the size of the pupil, the red cheek on
the affected side in pneumonia, the occurrence of glycosuria during
sciatica,—are all instances of such reflex acts. Many vaso-motor
affections are produced by irritation causing reflex effects at a
distance from the seat of irritation—a fact always to be kept in mind.
The utility of counter-irritation to the surface in diseases of the
internal organs is explained by supposing that vascular changes are
produced in those {1249} organs through reflex mechanisms set in
action by the local irritation. While some of these reflexes may have
their central mechanism in the local ganglia, it is probable that the
majority are to be traced to the spinal cord. It is believed that the
sympathetic ganglia are not the seat of reflex centres.
[Illustration: FIG. 57. Diagram to represent the Mode of Action of
Counter-irritants applied to the Chest (Lauder Brunton). The
irritation of the afferent nerves by the blister on the chest wall
gives rise to a vaso-constrictor reflex in the vessels of the lung.]
Since the vaso-motor nerves are connected almost exclusively with the
dorsal portion of the spinal cord, it is very natural to conclude that
the vaso-motor reflex centres are situated in this region; and the
hypothesis has been advanced by Jacubovitch, and strongly urged by
Gaskell,[11] that the cells of the vesicular columns of Clarke, which
are peculiar to this region, are the seat of these reflex mechanisms.
This hypothesis gains some support from the pathology of
syringo-myelia. In this disease the gray matter surrounding the
central canal and the vesicular columns are destroyed. The
characteristic symptoms are vaso-motor and trophic disturbances,
consisting of changes in the vascular tone, changes of local
temperature, and various eruptions, in some cases going on to
ulceration in the skin and mucous membranes. It is, however, undecided
whether the vaso-motor centres of the cord are limited to the columns
of Clarke, or are situated in the gray matter surrounding the central
canal, since both these parts are destroyed in this disease.[12] That
they are not located in the anterior or posterior gray cornua is
determined by the fact that diseases limited exclusively to these
areas do not cause vaso-motor disturbances. The situation of the
various reflex centres for the various parts of the body is at
different levels of the cord, as has been determined by the
experiments already cited to establish the level of origin of the
vaso-motor nerves. The exact location of the vaso-constrictor and
vaso-dilator reflex centres for definite parts is yet to be
ascertained.
[Footnote 11: _Loc. cit._]
[Footnote 12: See Fürstner, _Arch. für Psych._, xiv. 422.]
Vaso-motor Tracts.—These reflex centres are connected with the medulla
by tracts which lie in the lateral columns of the spinal cord,[13]
although it is not determined in which part of these columns. It is
not possible as yet to separate the constrictors from the dilators in
this tract, nor to determine whether it transmits impulses in both
directions or only from above downward. Nor is the course of
associating fibres between reflex centres at different levels known.
In cases of transverse myelitis the control of the medulla is removed
from the vascular centres below the lesion, and the lack of vascular
tone seen in the paralyzed limbs, together with the susceptibility to
local irritation, is the result of this division of the vaso-motor
tracts.
[Footnote 13: “Owsjanikow and Tschirijew,” _Bull. de l'Acad. de St.
Petersbourg_, xviii. 18.]
{1250} Medullary Centres.—It has been stated already that a general
vaso-motor centre with both constrictor and dilator powers is situated
in the medulla. This lies in two divisions on each side of the middle
line, in or just beneath the floor of the fourth ventricle, from the
calamus scriptorius up to the level of the sixth nerve-nucleus. Each
division governs the vascular tone of its own side of the body,[14]
and lesions in its region in man produce unilateral vaso-motor
symptoms.[15] This centre can be excited to reflex action by strong
irritation locally or through the blood, in which case a general
constriction or dilatation of the vessels of the entire body will
ensue. It seems probable, however, that the general centre in the
medulla is made up of a number of special centres, each of which
governs a definite set of organs. The vascular tone of the thoracic
and abdominal viscera is certainly regulated by a series of such
centres. Brown-Séquard and Schiff have produced hemorrhages in the
lungs, pleura, stomach, intestines, and kidneys at different times by
destructive lesions of the medulla, and the well-known experiments of
Bernard, in which by puncture of the medulla local hyperæmia of the
liver or kidneys was caused, producing glycosuria or polyuria, confirm
this view. Lesions of these parts in man produce similar effects.
Charcot has shown that in cerebral hemorrhage ecchymoses may be found
in the stomach, pleura, and endocardium, and that pneumonia is
especially frequent upon the paralyzed side. De Jonge[16] has been
able to collect thirteen cases of diabetes mellitus in which a lesion
of the medulla (hemorrhage or tumor) was found after death; and
Flatten[17] has proven the existence of similar lesions in diabetes
insipidus. The connection of these centres with the liver and kidneys
has been traced elsewhere.[18] The medulla contains a special centre
for the vaso-motor nerves of the abdomen, which are in the domain of
the splanchnic nerves. This centre is excited reflexly by impulses
reaching it through the depressor nerve of Cyon from the heart; so
that when that organ is overburdened it may be relieved by a fall of
arterial pressure produced by dilatation of the abdominal vessels.
Whether the connection of the medulla with the centres in the
semilunar ganglion which preside directly over these vessels is made
by way of the spinal cord or by way of the pneumogastric nerve is
still undetermined, though the researches of Gaskell favor the former
view. Gastric and intestinal disturbances are certainly produced by
nervous lesions in the medulla, but whether they are due to vascular
changes is uncertain. The vomiting of mucus and blood, and the large
watery evacuations which accompany mental shock or anxiety, as well as
the polyuria associated with mental effort, have been ascribed to
irritation of local centres in the medulla governing the
gastro-intestinal and urinary organs by impulses received from the
cortex above. The spleen is under the control of vaso-motor centres,
since section of the splenic branches of the semilunar ganglia will
produce a great enlargement of the organ, and irritation of the cut
end of these branches will produce {1251} contraction.[19] The medulla
also contains a vaso-dilator centre for the erectile tissues of the
genital organs, irritation of which by mental action or local disease
causes impulses to pass to the nervi erigentes by way of the spinal
cord, resulting in a flow of blood to the parts. Although a centre has
been thought to exist controlling the circulation in the lungs, whose
paralysis has been supposed to explain the occurrence of sudden
pulmonary œdema without other known cause, no definite facts regarding
it are known. That the action of the heart is under the control of the
medulla is a fact too well known to require more than a mention. The
physiology of the nervous control of the heart cannot be discussed
here.
[Footnote 14: Owsjanikow, _Arbeiten aus d. Physiol. Instit. zu
Leipzig_, 1871.]
[Footnote 15: M. A. Starr, “Sensory Tract in Central Nervous System,”
_Journ. Nerv. and Ment. Dis._, July, 1884, pp. 396-398.]
[Footnote 16: _Arch. f. Psych._, xiii.]
[Footnote 17: _Ibid._]
[Footnote 18: See Tyson, “Diabetes Mellitus,” _Pepper's System of
Medicine_, Vol. I. p. 195; Edes, “Diabetes Insipidus,” _ibid._, Vol.
IV. p. 30.]
[Footnote 19: Tarchanoff, _Pflüger's Arch._, viii. p. 97; Ross,
_Diseases of the Nervous System_, vol. i. p. 225.]
While these medullary centres are certainly influenced by impulses
reaching them from the cerebral hemispheres, as is evident from the
vaso-motor symptoms produced by mental action—_e.g._ pallor from
fright, blushing, etc.—it is impossible to state in what portion of
the hemispheres in man the higher vaso-motor centres lie. Eulenburg
and Landois locate them in the motor area in animals.[20] They are
certainly beyond control of the will, and are wholly reflex in their
action, a purely mental act in this case being the excitant of a
purely physical result.[21]
[Footnote 20: _Arch. f. Path. Anat._, Bd. lxviii. p. 245.]
[Footnote 21: In addition to the articles already cited the reader is
referred to Landois's _Physiology_, to Duval's article,
“Vaso-moteurs,” in the _Dictionnaire de Médecine et de Chirurgie_,
vol. xxxviii. (1885), for a summary of vaso-motor physiology, and to
Gerhardt's “Ueber Angio-neurosen,” _Volkmann's Sammlung klin.
Vorträge_, No. 209. Gaskell's researches, published in the _Journal of
Physiology_, are the most recent and satisfactory.]