Arthur Sheridan Lea Sir Michael Foster.

A text book of physiology online

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tribution of those continued constrictor impulses by which the
general arterial tone of the body is maintained, and that an
increase or decrease of vaso-con stricter action in particular arteries,
or in the arteries generally, is brought about by means of the same
bnlbar vaso-motor centre. But we must not therefore conclude
that this small portion of the spinal bulb is the only part of
the central nervous system which can act as a centre for vaso-con-
strictor fibres ; and, so we have seen, there is no evidence at
piresent that the vaso-dilator fibres are connected with either this
or any other one centre. In the frog reflex vaso-motor effects may
be obtained by stimulating various afferent nerves after the whole
spinal bulb has been removed, and, indeed, even when only a com-
paratirely small portion of the spinal cord has been left intact, and
ooanected, on the one hand, with the afferent nerve which is being
stimalated, and, on the other, with the efferent nerves in which

the vaso-motor fibres, whose action is being studied. In the
such effects do not so readily appear, but may with care



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284 SUMMARY OF VASO-MOTOR ACTIONS. [Book i.

and under special conditions be obtained. Thus in the dog, when
the spinal cord is divided in the thoracic region, the arteries of
the hind limbs and hinder part of the body, as we have already
said, § 150, become dilated. This one would naturally expect as
the result of their severance from the bulbar vaso-motor centre.
But if the animal be kept in good condition for some time, a
normal or nearly normal arterial tone is after a while re-estab-
lished; and the tone thus regained may, by afferent impulses
reaching the cord below the section, be modified in the direction
certainly of diminution, i e. dilation, and possibly, but this is not
so certain, of increase, L e, constriction ; dilation of various cutane-
ous vessels of the limbs may be readily produced by stimulation
of the central stump of one or another nerve.

These and other results lead to the conclusion that the bulbar
vaso-motor centre is not to be regarded as the sole vaso-motor
centre, whence alone can issue tonic constrictor impulses or
whither afferent impulses from this or that part of the body must
always travel before they can affect the vaso-constrictor impulses
passing along this or that nerve. We are rather to suppose that
the spinal cord along its whole length contains, interlaced with
the reflex and other mechanisms by which the skeletal muscles
are governed, vaso-motor centres and mechanisms of varied com-

{)lexity, the detdls of whose functions and topography have yet
argely to be worked out. As in the absence of the sinus venosus
the auricles and ventricle of the frog's heart may still continue to
beat, so in the absence of the spinal bulb these spinal vaso-motor
centres provide for the vascular emergencies which arise.

§ 156. We may sum up the history of vaso-motor actions
somewhat as follows.

In the case of at least a very large number of the arteries of
the body we have direct experimental evidence that these arteries
are connected with the central nervous system by nerve fibres,
called vaso-motor fibres, the action of which varies the amount of
contraction of the muscular coats of the arteries and so leads to
changes in calibra The action of these vaso-motor fibres is more
manifest, and probably more important in the case of small and
minute arteries than in the case of large ones.

These vaso-motor fibres are of two kinds. The one kind, vaso-
constrictor fibres, are of such a nature or have such connections
at their peripheral endings that stimulation of them produces
narrowing, constriction of the arteries. During life these fibres
appear to be the means by which the central nervous system
exerts a continued tonic influence on the arteries and maintains
an arterial * tone ; ' and this arterial tone may be modified by the
action of the central nervous system, so as to give place on the
one hand to constriction and on the other to widening. The other
kind, vaso-dilator fibres, are of such a kind, or have such conneo-
tions, that stimulation of them produces widening, dilation of the



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CHkP. TY.'\ THE VASCULAR MECHANISM. 285

wtftTies. Tliere is no adequate evidence that these vasodilator
fibres serve as channels for tonic dilating impulses or influences.

The vaso-constrictor fibres leave the spinal cord by the anterior
roots of the nerves coming from the middle region only of the
spinal cord. In the dog, this region extends from about the first
or second thoracic to the fourth or fifth lumbar nerve ; and in
other animals is probably of corresponding extent. Leaving the
spinal nerves by the respective visceral branches, rami communi-
cantes, the fibres pass into the sympathetic system, the majority
joining the main sympathetic chain of ganglia in the thorax and
abdomen, but some, for instance those going to certain parts of
the intestine and some other viscera, leaving that chain on one
side and passing directly to more peripheral ganglia, such as the
solar plexus and the inferior mesenteric ganglia. From the sym-
pathetic chain the fibres run to their destination in such nerves
as the cervical sympathetic and splanchnic, those allotted to the
skin of the limbs and trunk running back again to join the respec-
tive spinal nerves. In the ganglia of the sympathetic chain or in
other more peripheral ganglia the fibres lose their medulla, and
continue their course as non-medullated fibres.

In the intact organism the emission and distribution along
these vaso-constrictor fibres of tonic constrictor impulses, by which
general and local arterial tone is maintained and regulated, is
governed by a limited portion of the spinal bulb known as the
bulhar vaso-motor centre ; and when some change of conditions or
other natural stimulus brings about a change in the activity of the
vaso-constrictor fibres of one or more vascular areas, or of all the
sjteries suppUed with vaso-constrictor fibres, this same bulbar
Taso-motor centre appears in such cases to play the part of a
centre of reflex action. Nevertheless, in cases where the nervous
connections of this bulbar vaso-motor centre with a vascular area
ue cut off by an operation, as by section of the cord, other parts
of the spinal cord may act as centres for the vaso-constrictor
fibres of the area, and possibly these subordinate centres may be
to a certain extent in action in the intact organism.

The vaso-dilator fibres of whose existence we have clear and
undisputed experimental evidence, are very limited in distribution.
In the cases best known, the fibres leave certain regions of the
central nervous system and proceed to their destination along
certain cerebro-spinal nerves ; they do not lose their medulla
nntil they approach their termination. But as we have seen there
is evidence of vaso-dilator fibres also running in nerves of the
sympathetic system. The vaso-dilator fibres are generally thrown
into action as part of a reflex act, and the centre, in the reflex act,
appears in each case to he in the central nervous system not far
frcnn the origin of the ordinary motor fibres which the dilator
fibres accompany.

The effects of the activity of the vaso-dilator fibres appear to be



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286 EXAMPLES OF VASO-MOTOR ACTIONS. [Book l

essentially local in nature. When any set of the fibres come into
action the vascular area which these govern is dilated ; and the
vascular areas so governed are relatively so small that changes in
them produce little or no effect on the vascular system in general ;
the fibres are called into play to produce special effects in special
organs.

The effects of changes in the activity of the vaso-constrictor
fibres are both local and general. They are also double in nature ;
by an inhibition of tonic constrictor impulses a certain amount of
dilation may be effected ; by an augmentation of constrictor im-
pulses, constriction, it may be of considerable extent, may be
brought about. When the vascular area so affected is small the
effects are local, more or less blood is distributed through the area ;
when the vascular area affected is large, the inhibition of constric-
tion may lead to a marked fall, and an augmentation of constric-
tion to a marked rise of general blood pressure. Broadly speaking,
we may say that whenever a vascular change is needed for the
general well-being of the economy, it is this vaso-constrictor
system which is called into play.

The distribution of clearly proved vaso-dilator fibres is as we
have said very limited, and even the vaso-constrictor fibres are
most abundant in the nerves going to the skin and to the viscera.
In respect to the arteries supplying the numerous skeletal mus-
cles, there is much dispute as to whether they are supplied by
vaso-dilator fibres; and the supply of vaso-constrictor fibres to
them is at least not large. We may peihaps infer that the vascu-
lar changes in the muscles are intended chiefly for the benefit of
the muscles themselves, and are not to any great extent, like those
of the skin and viscera, utilized for the more general purposes of
the economy.

§ 157. We shall have occasion later on again and again to
point out instances of the effects of vaso-motor action both local
and general, but we may here quote one or two characteristic
examples. " Blushing " is one. Nervous impulses started in
some parts of the brain by an emotion produce a powerful inhibi-
tion of that part of the bulbar vaso-motor centre which governs
the vascular areas of the head supplied by the cervical sympa-
thetic, and hence has an effect on the vaso-motor fibres of the
cervical sympathetic almost exactly the same as that produced by
section of the nerve. In consequence the muscular walls of the
arteries of the head and face relax, the arteries dilate and the
whole region becomes suffused. Sometimes an emotion gives rise
not to blushing, but to the opposite effect, viz. to pallor of the face.
In a great number of cases this has quite a different cause, being
due to a sudden diminution or even temporary arrest of the heart's
beats ; but in some cases it may occur without any change in the
beat of the heart, and is then due to a condition the very converse
of that of blushing, that is, to an increased arterial constriction ;



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Chap. 1V.3 THE VASCULAR MECHANISM. 287

and this increased constriction, like the dilation of blushing, is
effected through the agency of the central nervous system and the
cervical sympathetic. Blushing and its opposite pallor are most
marked in the face ; but other parts of the body may blush (or
grow pale) the change being brought about by appropriate nerves.

The vascular condition of the skin at large affords another
instance. When the temperature of the air is low the vessels of the
skin are constricted, and the skin is pale ; when the temperature of
the air is high the vessels of the skin are dilated, and the skin is
red and flushed. In both these cases the effect is mainly a reflex one,
it being the central nervous system which brings about augmen-
tation of constriction in the one case and inhibition in the other ;
though possibly some slight effect is produced by the direct local
action of the cold or heat on the vessels of the skin. Moreover
the vascular changes in the skin are accompanied by corresponding
vascular changes in the viscera (chiefly abdominal) of a reverse
kiniL When the vessels of the skin are dilated those of the
viscera are constricted, and vice versa; so that a considerable
portion of the whole blood ebbs and flows, so to speak, according
to circumstances from skin to viscera and from viscera to skin.
By these changes, as we shall see later on, the maintenance of the
normal temperature of the body is in large measure secured.

We shall see later on that the secretion of urine is in a peculiar
way dependent on the flow of blood through the kidney. A vgry
fiivoarable condition for this flow is a dilated condition of the renal
arteries coincident with a high general blood pressure, and this
coodition as we shall see leads to a copious secretion of urine.
The high general blood pressure in this case is largely caused
by very g3neral arterial constriction, leading to great increase
of peripheral resistance, while the dilated state of the renal arteries
appear") to be due to a lack of the usual tonic constrictor impulses ;
though these constrictor impulses are increased in respect to other
arteries, they are diminished in respect to the renal arteries
them -selves.

When food is placed in the mouth the blood vessels of the
silivary glands as we have seen are flushed with blood as an
arljuvant to the secretion of digestive fluid; and as the food
passes along the alimentary canal each section in turn, with
the glandular appendages belonging to it, welcomes its advent by
flushing with blood. As we have already said, we have, at present,
no satisfactory evidence, except in the case of the salivary glands,
that this flu«?hing is carried out by special vaso-dilator nerves. Along
the rest of the alimentary canal the widening of the arteries and
thus the increased flow seems to be brought about by diminution
of vaao-constrictor impulses, so far at least as it is ensured by the
intervention of the central nervous system. We say ' so far'
because as we shall see we have evidence that the vessels of the
kidney may change in caHbre independently of any influences



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988 VASO-MOTOR NERVES OF THE VEINS. [Booki.

proceeding from the central nervous system, after for instance all
the nerves going to the kidney have been divided ; in such cases
the changes in the calibre of the renal vessels seem to be due to
some direct local action ; and it is possible that the flushing of the
alimentary canal when food enters it is similarly, in part or at
times, the result of some local action on the blood vessels.

§ 158. Vaso-motornerves of the Veins. Although the veins are
provided with muscular fibres and are distinctly contractile, and
although rhythmic variations of calibre due to contractions may
be seen in the great veins opening into the heart, in the veins of
the bat*s wing, and elsewhere, our knowledge as to any nervous
arrangements governing the veins is at present very limited. The
portal vein, the walls of which are conspicuously muscular, the
muscular fibres being arranged both as a circular and as a longi-
tudinal coat, is like the veins just mentioned subject to rhythmic
variations of calibre; these might be due to active rhythmic
contractions of the portal vein itself or might be of a passive
nature, due to a rhythmic rise and fall in the quantity of blood
discharged into it from the vessels of the viscera. The former
view is supported by the observation that after the aorta has been
obstructed, so that no blood can pass into the portal vein from the
mesenteric and other arteries, contractions of the portal vein may
be obtained by stimulating the splanchnic nerves. The great
distention of the venous system with blood which occurs in the
frog when the brain and spinal cord are destroyed, and which
renders the heart almost bloodless, the greater part of the blood
being lodged in the veins, has also been supposed to point to some
normal tone of the veins dependent on the central nervous
system.



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SEC. 7. THE CAPILLARY CIRCULATION.



§ 159. We have already, some time back (§ 99), mentioned
some of the salient features of the circulation through the capil-
laries, viz. the difficult passage of the corpuscles (generally in
single file, though sometimes in the larger channels two or
more abreast) and plasma through the narrow channels, in a
stream which though more or less irregular is steady and even, not
broken by pulsations, and slower than that in either the arteries
« the veins. We have further seen (§ 94) that the capillaries
vary very much in width from time to time ; and there can be
no doubt that the changes in their calibre are chiefly of a passive
nature. They are expanded when a large supply of blood reaches
them through the supplying arteries, and, by virtue of their
elasticity, shrink again when the supply is lessened or withdrawn ;
they may also become expanded by an obstacle to the venous
outflow.

On the. other hand, there is a certain amount of evidence that,
in young animals at all events, the calibre of a capillary canal
may vary, quite independently of the arterial supply or the
venofu outflow, in consequence of changes in the form of the
epithelioid cells, allied to the changes which in a muscle-fibre or
miwcle-cell constitute a contraction ; and though the matter re-
quires further investigation, it is possible that these active changes
play an important part in determining the quantity of blood pass-
ing? through a capillary area ; but there is as yet no satisfactory
cTideoce that they, like the corresponding changes in the arteries,
are governed by the nervous system.

Over and above these changes of form, the capillaries and
mbute vessels are subject to still other changes and so exert
mfluences by virtue of which they pla/ an important part in the
wotk of the circulation. Their condition determines the amount
ol resistance offered by their channels to the flow of blood through
those channels, and determines the amount and character of that
interchange between the blood and the tissues which is the main
faa of the circulation.

19



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290 INFLAMMATION. [Book i.

If the web of the frog's foot, or, better still, if some transparent
tissue of a mammal be watched under the microscope, it will be ob-
served that, while in the small capillaries the corpuscles are pressed
through the channel in single file, one after the other, each corpuscle
as it passes occupying the whole bore of the capillary, in the larger
capillaries (of the mammal), and especially in the small arteries
and veins which permit the passage of more than one corpuscle
abreast, the red corpuscles run in the middle of the channel, forming
a coloured core, between which and the sides of the vessels all
round is a colourless layer, containing no red corpuscles, called
the 'plasmatic layer' or 'peripheral zone.* This division into a
peripheral zone and an axial stream is due to the fact that in any
stream passing through a closed channel the friction is greatest
at the sides, and diminishes towards the axis. The corpuscles
pass where the friction is least, in the axis. A quite similar axial
core is seen when any fine particles are driven with a suflBcient
velocity in a stream of fluid through a narrow tube. As the
velocity is diminished the axial core becomes less marked and
disappears.

In the peripheral zone, especially in that of the veins, are
frequently seen white corpuscles, sometimes clinging to the sides
of the vessel, sometimes rolling slowly along, and in general moving
irregularly, stopping for a while and then suddenly moving on.
The greater the velocity of the flow of blood, the fewer the white
corpuscles in the peripheral zone, and with a very rapid flow they,
as well as the red corpuscles, may be all confined to the axial
stream. The presence of the white corpuscles in the peripheral
zone has been attributed to their being specifically lighter than
the red corpuscles, since when fine particles of two kinds, one lighter
than the other, are driven through a narrow tube, the heavier
particles flow in the axis and the lighter in the more peripheral
portions of the stream. But, besides this, the white corpuscles
have a greater tendency to adhere to surfaces than have the red,
as is seen by the manner in which the former become fixed to
the glass slide and cover-slip when a drop of blood is mounted
for microscopical examination. They probably thus adhere by
virtue of the amoeboid movements of their protoplasm, so that the
adhesion is to be considered not so much a mere physical as a
physiological process, and hence may be expected to vary with the
varying nutritive conditions of the corpuscles and of the blood
vessels. Thus while the appearance of the white corpuscles in the
peripheral zone may be due to their lightness, their temporary
attachment to the sides of the vessels and characteristic progression
is the result of their power to adhere ; and as we shall presently
see their amoeboid movements may carry them on beyond mere
adhesion.

§ 160. These are the phenomena of the normal circulation,
and may be regarded as indicating a state of equilibrium between



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Chap, iv.] THE VASCULAR MECHANISM. 291

the blood on the one h&nd and the blood vessels with the tissues
on the other ; but a dififerent state of things sets in when that
equilibrium is overthrown by causes leading to what is called
inflammation or to allied conditions.

If an irritant, such as a drop of chloroform or a little diluted
oQof mustard, be applied to a small portion of a frog's web, tongue,
mesentery, or some other transparent tissue, the following changes
may be observed under the microscope ; they may be still better seen
itt the mesentery or other transparent tissue of a mammal. The
first effect that is noticed is a dilation of the arteries, accompanied
by a quickening of the stream. The irritant, probably by a direct
action on the muscular fibres of the arteries, has led to a re-
laxation of the muscular coat, and hence to a widening ; and we
have already, § 105, explained how such a widening in a small
artery may lead to a temporary quickening of the stream. In
consequence of the greater flow through the arteries, the capillaries
become filled with corpuscles, and many passages, previously
invisible or nearly so on account of their containing no corpuscles,
now come into view. The veins at the same time appear enlarged
and fulL If the stimulus be very slight, this may all pass away,
the arteries gaining their normal constriction, and the capillaries
and veins returning to their normal condition ; in other words, the
effect of the stimulus in such a case is simply a temporary blush.
Unless, however, the chloroform or mustard be applied with especial
care, the effects are much more profound, and a series of remarkable
changes set in.

In the normal circulation, as we have just said, white corpuscles
may be seen in the peripheral, plasmatic zone, but they are scanty
in number, and each one, after staying for a little time in one spot,
suddenly gets free, sometimes almost by a jerk as it were, and then
rolls on for a greater or less distance. In the area now under
consideration a large number of white corpuscles soon gather in
tlie peripheral zones, especially of the veins and venous capillaries
ithat is of the larger capillaries which are joining to form veins),
Itot also, of the other capillaries, and, to a less extent, of the arteries ;
and this takes place although the vessels still remain dilated and the
stream still continues rapid, though not so rapid as at first Each
^vrhite corpuscle appears to exhibit a greater tendency to stick to
the sides of the vessels, and though driven away from the arteries
by the stronger arterial stream, becomes lodged so to speak in the
veins. Since new white corpuscles are continually being brought
by the blood stream on to the scene, the number of them in the
peripheral zones of the veins increases more and more, and this
msT go on until the inner surface of the veins and venous
cs{Allaries appears to be lined with a layer of white corpuscles.
The small capillaries too contain more white corpuscles than
usual, and even in the arteries these are abundant, though not
forming the distinct layer seen in the veins. The white cor«



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292 MIGRATION OP WHITE CORPUSCLES. [Book i-

puscles, however, are not the only bodies present in the peri-
pheral zone. Though in the normal circulation blood-platelets
(see § 33) cannot be seen in the peripheral zone, and hence (on
the view, which has the greater support, that these bodies are
really present in quite normal blood) must be confined to the axial
stream, they make their appearance in that zone during the
changes which we are now describing. Indeed, in many cases they
are far more abundant than the white corpuscles, the latter appear-*
ing imbedded at intervals in masses of the former. Soon after
their appearance the individual platelets lose their outline, and run
together into formless masses.



Online LibraryArthur Sheridan Lea Sir Michael FosterA text book of physiology → online text (page 35 of 148)