and inferior vena cava open into the right auricle of
The course of the blood through the chambers of
the fetal heart differs from that observed in the adult,
viz., the blood from the inferior cava passes into the
right auricle and then is directed by the Eustachian
valve to the left auricle of the heart through the
In the left auricle the blood from the
right auricle becomes mixed with a small quantity
of blood returned from the lungs by the pulmonary
veins. This blood then passes into the left ventricle
(as seen in the adult) and then into the aorta, by which
vessel it is distributed almost entirely to the head and
upper extremities. The blood is returned from the
head and upper extremities by the veins, w r hich
ultimately drain into the superior vena cava, that
Duct us aneriomts
lnterna I iliac arteries.
Plan of the Fetal Circulation.
In this plan the figured arrows represent the kind of blood, as
well as the direction which it takes in the vessels. Thus, arterial
blood is figured >^ >; venous blood, >; mixed
(arterial and venous) blood, >.
THE CIRCULATION OF THE BLOOD 185
opens into the right auricle of the heart. From the
right auricle the blood passes over the Eustachian
valve into the right ventricle, then from the latter
into the pulmonary artery. The lungs of the fetus
being inactive, require only enough blood to develop
and nourish them, this quantity of blood is conveyed
by the pulmonary arteries and returned to the pul-
monary veins to the left auricle; the greater quantity
of blood from the right ventricle passes through the
ductus arteriosus (a small vessel connecting the pul-
monary artery with the aorta) into the beginning of
the descending aorta, where it becomes mixed with
the blood from the left ventricle.
The general distribution of blood through the body
of the fetus is similar to the adult system, with the
exception of the above-mentioned differences and the
return of waste materials from the fetus to the placenta
by way of the hypogastric arteries, which are branches
of the internal iliacs and join the umbilical vein at
the umbilicus (navel) to assist in forming the umbilical
cord of the fetus.
The Changes in the Circulatory Apparatus following
Birth. With the commencement of normal respiration
(the umbilical cord being ligated), the placental circu-
lation is cut off. Anywhere within ten days after
birth the foramen ovale closes and may continue as
a slight opening and not be injurious to health until
the final closure. When the foramen does not close,
the skin, etc., becomes cyanotic, due to the mixing of
the arterial and venous blood the so-called blue baby.
The umbilical vein, ductus venosus, and arteriosus
atrophy, and also the hypogastric arteries. Thus the
infant vascular apparatus assumes the normal course
as described in the adult.
Structure and Functions of the Arteries, Veins, and
Capillaries. The Arteries. These are closed tubes
which convey the blood and keep it in circulation
from the heart cavities throughout the body.
THE CIRCULATORY APPARATUS
The typical artery consists of three coats: internal
or tunica intima; middle or tunica media; an external
or tunica adventitia. Aside from the latter, most
arteries are covered by a sheath of connective tissue
derived from the fascia of the part of the body in
which they are found, and held to the artery by
Transverse section of part of the wall of the posterior tibial artery. X 75.
A', endothelial and subendothelial layers of inner coat. B, elastic layer (fenes-
trated membrane) of inner coat, appearing as a bright line in section. C,
muscle layer (middle coat). D. outer coat, consisting of connective-tissue
bundles. In the interstices of the bundles are some connective- tissue nuclei,
and, especially near the muscular coat, a number of elastic fibers cut across.
The muscular tissue (media) is not so well-marked in
the larger arteries, but the elastic tissue predominates
and is more closely arranged; in the smaller arteries
the elastic tissue is in excess, while only a single
layer of muscle tissue is present; the larger arteries
possess a thicker outer or fibrous coat than the smaller
All arteries possess elasticity and contractility due
to the presence in their walls of the elastic (intima)
and muscle (media) coats.
Elasticity. This elastic property possessed by
arteries permits their wall to expand and recoil to
adapt itself to the pressure and reaction of the column
of blood thrown into the arterial system from the
ventricle of the heart at each contraction or systole
and subsequent relaxation or diastole.
THE CIRCULATION OF THE BLOOD 187
This elasticity of the arterial wall, in response to
the pressure and reaction of the column of blood
imparted to it by the heart muscle, develops into a
remittent expansion and recoil of the arterial wall,
which becomes fainter the more distant the vessels
are from the heart. When the capillaries are reached
it is a continuous or a steady flow of blood, without
any recoil of the arterial wall, which passes into the
veins. Thus the elasticity of the arteries is for the
purpose of equalizing the movement of the blood
throughout the arterial system.
Contractility. Contractility of the arteries is depen-
dent upon the muscular tissue in their walls. They
are supplied by nerve filaments which receive impulses
from the controlling centres in the spinal cord, which
communicate by means of ganglia with the sympathetic
system, and the latter distributes filaments to the
middle or muscular coat of the arteries. The centres
in the spinal cord are called the vasomotor centres, and
the nerve filaments the vasomotor nerves. The centres
in the spinal cord are influenced chiefly by a main
centre situated in the medulla; in other words, the
spinal centres are underlying centres of this system.
The vasomotor nerves possess two sets of fibers:
those which when stimulated raise the blood pressure
or contract the arterial wall called vasoconstrictor
nerves; and those which lower arterial pressure or
dilate the arterial wall called vasodilator nerves.
These two sets of nerves, both when active although
antagonistic to each other in function, tend to keep
the arteries in a normal state of contraction, thus
regulating the blood pressure and caliber of the
The vasa vasorum is the term defining the blood-
vessels which supply the walls of the arteries. They are,
of course, very minute bloodvessels which nourish
them, and are derived from adjacent branches and have
no direct opening into the channel of the artery they
THE CIRCULATORY APPARATUS
supply. They are located in the tunica media or
middle coat. The lining coat or intima of an artery
is nourished by the column of blood passing over it.
The arteries give off branches which become smaller
as their destination the capillary area of the cir-
culation is reached. Just before
emerging into capillaries, these small
branches are termed arterioles.
The Capillaries. The capillaries
consist of a series of minute blood-
vessels which divide and subdivide
to form interlacing net-works. The
walls of a capillary consist of a single
layer of flattened endothelial cells,
nucleated, and held together by a
cement substance. These thin-walled
vessels possess irritability, contrac-
tility, and tonicity. They freely
communicate with one another, and
are the connecting link for the pas-
sage of the blood from the arteries
to the veins. They are just large
enough to transmit a small red cell.
The functions of the capillary wall
is to allow an exchange of the nutri-
tive elements of the blood with the
tissues, and to receive from the tissue
the waste products arising as a result
of metabolism, taking place in the
protoplasm of the tissue cells. This
interchange between the blood and the tissues is
due to the thinness of the capillary wall, aided by
the phenomena of diffusion, osmosis, and infiltration
(see Absorption, page 286).
The Veins. The veins consist of three coats: the
tunica intima, media, and adventitia. They differ
from the arteries by their middle coat possessing
less elastic and muscular tissue, but an increase in the
Capillaries from the
mesentery of a guinea-
pig after treatment with
a solution of nitrate of
silver: a, cells; b, their
THE CIRCULATION OF THE BLOOD 189
amount of fibrous connective tissue. They readily
collapse when empty, do not pulsate, and possess
elasticity and contractility, but not as marked as
seen in the arteries; however, these properties aid in
forcing the column of blood toward the heart, after
leaving the capillaries particularly after any obstruc-
tion to the flow of the blood stream. The veins collect
the blood from the capillary area throughout the
body, and return it to the right side of the heart.
They start as very small vessels called venules, and
then become veins, which increase in size as they
course toward their main trunks.
Veins possess valves, arranged in pairs, composed
of folds of the lining membrane intima. They always
project toward the heart, and are flattened against the
wall of the vessel if the blood is flowing unobstructed
toward the heart; but when any obstruction takes
place they are distended into the channel, preventing
a return flow, or regurgitation of the blood.
The veins distend under these conditions due to
their elasticity and accommodate the blood, until the
obstruction is removed, when the muscle fibers in
the wall contract and force the column of blood
onward. With the congestion relieved, the valves
again collapse against their walls.
The Pulse. The pulse is the regular beat which
is transmitted to the examining finger w r hen placed
on an artery. It is felt best in the radial artery by
making gentle pressure with the tip of the index
finger in the region of the artery at the lower fourth
of the forearm on the outer side when the palm
is turned upward or supinated, just a little above
and to the inner side of the prominence of the
styloid process of the radius. By pressing downward
the beat or pulsation can be felt. The artery lies
on a firm bed formed by the flexor longus pollicis,
and pronator quadratus muscles, beneath which is
the bone. The expansion of the artery is due to the
190 THE CIRCULATORY APPARATUS
response of the arterial system which is receiving
at every systole or contraction of the ventricle a
large volume of blood; the shrinkage of the artery is
due to the elastic recoil of the arterial system or their
walls upon the volume of blood, forcing it forward,
into, and through the arteries, during the diastole
or relaxation of the ventricles. The pulse or pulse
wave is simply a wave passing from the heart over
the arterial system, forcing the blood throughout the
body until the capillaries are finally reached, when
the column of blood passes into the venous system.
The number of beats of the pulse is 72 per minute,
and varies accordingly. (See Heart-beat, page 183.)
The pulse is influenced by the same factors which inter-
fere with the heart's action. The pulse is spoken of
as frequent or infrequent, depending whether it is
above or below the normal rate 72 per minute;
quick or slow according to the suddenness with which
it strikes the examining finger; hard or soft, tense or
easily compressible, depending on the resistance which
the artery offers to the compressing finger; large, full,
or small, depending on the amount of blood in the
arterial system at the time of examination.
The Blood-pressure. The blood-pressure or arterial
tension may be defined as the pressure exerted radially
or laterally by the moving blood-stream against the
sides of the vessels (Brubaker).
The blood -pressure is greatest in the aorta and
gradually lessens as the blood is forced through the
vessels and emerges into the arterioles and capillaries,
then passes through the venules into the veins.
At each contraction of the heart a large volume of
blood is thrown into the arterial system, which is
already engorged. This mass of blood in the arterioles
and capillaries must be forced along to accommodate
the next column of blood thrown from the ventricle,
to relieve the arterial system of its already over-
distended condition, and maintains an even distribution
DESCRIPTION OF THE ARTERIES 191
of blood through the vascular system. Owing to the
small caliber of the arterioles the blood meets with
considerable resistance in passing through the arterioles.
As a result, there is a marked decrease in the pressure
in the arterioles and capillaries, due to this great
resistance, which is called the peripheral resistance.
The latter is caused by the small diameter of the
vessels modified by the tonic contraction of the
muscles in the wall of the arterioles.
A practical idea of the blood-pressure can be obtained
from observing a cut or injured bloodvessel. If a
large artery, the blood will be seen to project from
the cut end nearest the heart, as a bright red fluid
spurting from the vessel, with considerable force,
dependent on the degree of pressure which it had
been subjected to in the vessel, and the tension of
the vessel wall before the injury. A vein when injured
bleeds with no spurting or force. It is seen as a dark
blue fluid coming from the cut end away from the
heart, as a steady stream (welling up). These differ-
ences in the characters of the hemorrhage from an arte-
rial and venous course indicate the difference of blood-
pressure between the arterial and venous systems.
The venous pressure continues to fall from the
capillaries to the heart. There is simply a steady
tone to the walls of the veins which propel the blood
to the right side of the heart without any pulsation.
The capillary pressure is dependent on the blood-
pressure of the arterioles and venous systems. It
is too minute to observe under normal physiological
THE DESCRIPTION OF THE ARTERIES, THEIR
There are two great arterial systems: (1) The
pulmonary, to the lungs; (2) the corporeal, to every
other part of the body.
192 THE CIRCULATORY APPARATUS
The Pulmonary Arterial System. The pulmonary
artery is a short, wide vessel, 2 inches in length. Com-
mencing at the base of the right ventricle, it curves
upward and backward, to end under the transverse
aorta by dividing into a right and a left branch, which
convey the blood from the right auricle to the lungs.
This vessel, with the ascending aorta, is enclosed in a
sheath of pericardium. It winds around the aorta,
being at first in front, and later to the left side, of the
ascending portion. In fetal life the ductus arteriosus
connects it a little to the left of its division with the
Each branch enters the hilum of the corresponding
lung; the right, the larger, passing behind the ascending
aorta and superior vena cava; the left, in front of the
descending aorta. The left divides into two branches
for the lobes of the left lung; the right also divides
into two primary branches for the upper and lower
lobes. From the lower one of these is sent a branch
to the middle lobe. The pulmonary arteries are the
only arteries which carry venous blood.
The Corporeal Arterial System. The Aorta.
The aorta is the main trunk from which spring the
systemic arteries. From the base of the left ventricle
it runs upward, forward, and to the right as far as
the second right cartilage; then backward and to the
left, over the root of the left lung, to the fourth dorsal
vertebra; thence, along the spine, it descends through
the thorax and abdomen, to divide at the fourth
lumbar, into the common iliacs.
It has been divided, for convenience of description,
into the arch and the descending aorta. The arch is
subdivided into the ascending, transverse, and descend-
ing parts; the descending aorta, into the thoracic and
THE ARCH OF THE AORTA. The ascending part of
the arch runs upward, forward, and to the right, from
a point opposite the lower border of the third left
DESCRIPTION OF THE ARTERIES 193
The arch of the aoita and its branches. (Gray.)
194 THE CIRCULATORY APPARATUS
cartilage, to the upper border of the second right
cartilage. Close to its origin it presents three small
dilatations, the sinuses of Valsalva, indicating the
situation of the semilunar valves.
The transverse part of the arch passes backward and
to the left as far as the left side of the body of the
fourth dorsal vertebra.
The descending part of the arch descends to the
lower border of the fifth dorsal vertebra, ending in
the thoracic aorta.
THE BRANCHES OF THE ARCH OF THE AORTA. The
branches of the arch are five coronary, right and
left, from the ascending part; and the innominate,
left carotid, and left subclavian, from the transverse
part. The descending part gives off no branches.
The coronary arteries supply the heart and the coats
of the great vessels.
The innominate (brachiocephalic) artery is the largest
branch. It arises in front of the left carotid, and runs
obliquely to the right sternoclavicular joint, where
it divides into the right common carotid and right
The common carotid arteries are identical in course,
branches, and relations in the neck, but differ in
their origin. Thus, the right is a branch of bifurca-
tion of the innominate, while the left is a primary
branch of the transverse aorta.
The external carotid artery runs from the bifurcation
of the common carotid to the space between the
neck of the condyle of the mandible and the audi-
tory meatus, and there divides into the superficial
temporal and internal maxillary.
The internal carotid artery is a very tortuous vessel,
and at its origin is farther from the median line than
the external carotid, deriving the name "internal"
from its distribution. For description it is divided
into four parts: the first, or cervical; the second, or
petrous, is in the carotid canal; the third, or cavernous
DESCRIPTION OF THE ARTERIES
runs in the cavernous sinus; and the fourth, or cerebral,
is the terminal portion, and supplies a portion of the
brain and eye-ball.
Applied anatomy of the arteries of the neck, showing the carotid and sub-
clavian arteries. The hypoglossal nerve is not rightly placed in this drawing.
It forms the upper side of a triangle, the two lower sides of which are the
two bellies of the digastric. The lingual artery would then run under the
hyoglossus muscle, below the hypoglossal nerve. (Gray.)
THE CIRCULATORY APPARATUS
The subclavian arteries are divided into three parts,
the first running to the inner margin of the scalenus
anticus; the second, behind that muscle; the third,
from its outer border to the lower border of the first
3 r emporal branche
The ophthalmic artery and its branches., the roof of the orbit having been
rib, where it becomes the axillary artery. The right
and left vessels differ only in their first portions, the
right arising behind the sternoclavicular joint, from
the innominate; the left, from the transverse aorta as
a primary branch.
DESCRIPTION OF THE ARTERIES
The basilar artery, formed by the two vertebrals,
runs to the upper border of the pons, and divides into
the two posterior cerebrals. It gives off the following
(a) Several transverse arteries on each side. One
the auditory, enters the internal meatus; another, the
anteroinferior cerebellar, to the anterior border of the
The axillary artery and its branches. (Gray.)
(6) The superior cerebellar, to the upper surface,
joining the inferior cerebellar.
(c) The posterior cerebrals, to the under surface of the
posterior lobes of the cerebrum, receiving the posterior
communicating. They give off the posterior choroid
branches and supply the posterior perforated space.
198 THE CIRCULATORY APPARATUS
The circle of Willis is situated at the base of the
brain, it is an anastomosis formed by the bloodvessels
of the brain. The arteries entering into its formation
are: In front, the two anterior cerebral arteries,
branches of the internal carotid, which are connected
by the anterior communicating artery ; behind, by two
posterior cerebral arteries, branches of the basilar, and
these communicate latterly with the internal carotids
through the posterior communicating arteries.
The Arteries of the Upper Extermity. THE AXILLARY
ARTERY. The axillary artery is the continuation of
the subclavian. It extends from the lower border of
the first rib, where it is deeply placed, to the lower
border of the teres major tendon, where it is super-
ficial, and there becomes the brachial. It is described
in three parts the first, above the pectoralis minor;
the second, behind it; and the third, below it.
Branches. First part, superior and acromial thoracic;
second part, long and alar thoracic; third part, sub-
scapular and circumflex, posterior and anterior.
THE BRACHIAL ARTERY. The brachial artery extends
from the end of the axillary, at the lower border of
the teres major, to \ inch below the elbow-joint, divid-
ing into the radial and ulnar arteries.
The branches of the brachial artery are : The superior
profunda, nutrient artery, to the humerus, the inferior
profunda, anastomotica magna, and muscular..
THE RADIAL ARTERY. The radial artery runs from
the bifurcation of the brachial along the radial side
of the forearm to the wrist, and winds back to its pos-
terior surface. It then enters the palm through the
first dorsal interosseous, and runs across the hand to
form the deep palmar arch by joining the deep branch
of the ulnar, and gives off in the ball of the thumb the
superficialis volse, a branch which joins with the main
termination of the ulnar artery to form the superficial
DESCRIPTION OF THE ARTERIES L99
THE ULNAR ARTERY runs along the inner side of
the forearm to the wrist, crosses the annular ligament
The brachial artery. (Gray.)
and the palm of the hand, and joins the superficialis
volse to form the superficial arch.
200 THE CIRCULATORY APPARATUS
The radial and ulnar arteries (Gray.)
DESCRIPTION OF THE ARTERIES
The superficial palmar arch lies beneath the palmar
fascia and above the flexor tendons of the fingers.
The abdominal aorta and its branches (Gray.)
The deep palmar arch lies beneath the flexor tendons
and rests on the rnetacarpal bones and palmar inter-
THE CIRCULATORY APPARATUS
The Thoracic Aorta.-The thoracic aorta descends
from the lower border of the fifth to the front of the
last dorsal vertebra.
The celiac axis and its branches, the liver having been raised and the
lesser omentum removed.
The Abdominal Aorta. The abdominal aorta runs
from the last dorsal to the left side of the middle of
the fourth lumbar vertebra, there dividing into the
two common iliacs.
The branches of the thoracic aorta:
Nine pairs of intercostal arteries, two subcostal,
bronchial, esophageal, mediastinal, and pericardial.
DESCRIPTION OF THE ARTERIES 203
The branches of the abdominal aorta :
(a) Parietal and (b) visceral.
THE PARIETAL BRANCHES. (1) The phrenic, (2) the
lumbar, (3) the middle sacral.
THE VISCERAL BRANCHES. I. The celiac axis, i
inch long, divides into the gastric, hepatic, and splenic.
It is covered by the lesser omentum, rests below on
the pancreas; on each side is a semilunar ganglion and
on the right the lobus Spigelii, of the liver on the left
Branches. (a) The gastric artery runs to the cardiac
orifice of the stomach, thence to the right, along the
lesser curvature, in the lesser omentum as far as the
pylorus. It supplies both surfaces of the stomach and
the esophagus, anastomosing with the splenic, hepatic,
and esophageal arteries.
(6) The hepatic artery passes below the foramen of
Winslow to the pylorus, then ascends in the lesser
omentum, anterior to that foramen, and to the left
of the gall duct, to the transverse fissure of the liver,