PHYSIOLOGICAL ANATOMY.
CIRCULATORY ORGANS.
Having considered the formation of chyle, traced it through the
digestive process, seen its transmission into the _vena cava_, and,
finally, its conversion into blood, we shall now describe how it is
distributed to every part of the system. This is accomplished through
organs which, from the round of duties they perform, are called
_circulatory_. These are the Heart, Arteries, Veins, and Capillaries,
which constitute the _vascular system_.
Within the thorax or chest of the human body, and enclosed within a
membranous sac, called the _pericardium_, is the great force-pump of the
system, the heart. This organ, to which all the arteries and veins of
the body may be either directly or indirectly traced, is roughly
estimated to be equal in size to the closed fist of the individual to
whom it belongs.
It has a broad end turned upwards, and a little to the right side,
termed its _base_; and a pointed end called its _apex_, turned
downwards, forwards, and to the left side, and lying beneath a point
about an inch to the right of, and below, the left nipple, or just below
the fifth rib. Attached to the rest of the body only by the great
blood-vessels which issue from and enter it at its base, the heart is
the most mobile organ in the economy, being free to move in different
directions.
The heart is divided into two great cavities by a fixed partition, which
extends from the base to the apex of the organ, and which prevents any
direct communication between them. Each of these great cavities is
further subdivided transversely by a movable partition, the cavity above
each transverse partition being called the _auricle_, and the cavity
below, the _ventricle_, right or left, as the case may be.
[Illustration: Fig. 40.
General view of the heart and lungs, _t_. Trachea, or
windpipe, _a_. Aorta, _p_. Pulmonary artery, 1, 2.
Branches of the pulmonary artery, one going to the
right, the other to the left lung. _h._ The heart.]
The walls of the auricles are much thinner than those of the ventricles,
and the wall of the right ventricle is much thinner than that of the
left, from the fact that the ventricles have more work to perform than
the auricles, and the left ventricle more than the right.
In structure, the heart is composed almost entirely of muscular fibers,
which are arranged in a very complex and wonderful manner. The outer
surface of the heart is covered with the pericardium, which closely
adheres to the muscular substance. Inside, the cavities are lined with a
thin membrane, called the _endocardium_. At the junction between the
auricles and ventricles, the apertures of communication between their
cavities are strengthened by _fibrous rings_. Attached to these fibrous
rings are the movable partitions or valves, between the auricles and the
ventricles, the one on the right side of the heart being called the
_tricuspid valve_, and the one on the left side the _mitral valve._ A
number of fine, but strong, tendinous chords, called _chordæ
tendineæ_, connect the edges and apices of these valves with
column-like elevations of the fleshy substance of the walls of the
ventricles, called _columnæ carneæ_.
[Illustration: Fig. 41.
1. The descending vena cava. 2.
The ascending vena cava. 3. The
right auricle. 4. The opening between
the right auricle and the right
ventricle. 5. The right ventricle. 6.
The tricuspid valves. 7. The pulmonary
artery. 8, 8. The branches
of the pulmonary artery which pass
to the right and the left lung. 9. The
semilunar valves of the pulmonary
artery. 10. The septum between the
two ventricles of the heart. 11, 11.
The pulmonary veins. 12. The left
auricle. 13. The opening between
the left auricle and ventricle. 14.
The left ventricle. 15. The mitral
valves. 16, 16. The aorta. 17. The
semilunar valves of the aorta.]
The valves are so arranged that they present no obstacle to the free
flow of blood from the auricles into the ventricles, but if any is
forced the other way, it gets between the valve and the wall of the
heart, and drives the valve backwards and upwards, thus forming a
transverse partition between the auricle and ventricle, through which no
fluid can pass.
At the base of the heart are given off two large arteries, one on the
right side, which conveys the blood to the lungs, called the _pulmonary
artery_, and one on the left side, which conveys the blood to the system
in general, called the _aorta_. At the junction of each of these great
vessels with its corresponding ventricle, is another valvular apparatus,
consisting of three pouch-like valves, called the _semilunar valves_,
from their resemblance, in shape, to a half-moon. Being placed on a
level and meeting in the middle line, they entirely prevent the passage
of any fluid which may be forced along the artery towards the heart,
but, flapping back, they offer no obstruction to the free flow of blood
from the ventricles into the arteries.
[Illustration: Fig. 42.
A representation of the venous and arterial
circulation of the blood.]
The _Arteries_, being always found empty after death, were supposed by
the ancients, who were ignorant of the circulation of the blood, to be
tubes containing air; hence their name, which is derived from a Greek
word and signifies an _air-tube._ Arteries are the cylindrical tubes
which carry blood to every part of the system. All the arteries, except
the coronary which supply the substance of the heart, arise from the two
main trunks, the pulmonary artery and the aorta. They are of a
yellowish-white color, and their inner surface is smooth. The arteries
have three coats. (1.) The external coat, which is destitute of fat, and
composed chiefly of cellular tissue, is very firm and elastic, and can
readily be dissected from the middle coat. (2.) The middle, or fibrous
coat, is thicker than the external, and composed of yellowish fibers,
its chief property is contractility. (3.) The internal coat consists of
a colorless, thin, transparent membrane, yet so strong that it can, it
is thought, better resist a powerful pressure than either of the others.
Arteries are very elastic as well as extensible, and their chief
extensibility is in length. If an artery of a dead body be divided,
although empty, its cylindrical form will be preserved.
The _Veins_ are the vessels through which the venous blood returns to
the auricles of the heart. They are more numerous than the arteries, and
originate from numerous capillary tubes, while the arteries are given
off from main trunks. In some parts of the body, the veins correspond in
number to the arteries; while in others, there are two veins to every
artery. The veins commence by minute roots in the capillaries, which are
everywhere distributed through the body, and gradually increase in size,
until they unite and become large trunks, conveying the dark blood to
the heart. The veins, like the arteries, have three coats. The external,
or cellular coat, resembles that of the arteries; the middle is fibrous,
but thinner than the corresponding one of the arteries; and the internal
coat is serous, and analogous to that of those vessels. The veins belong
to the three following classes: (1.) The systemic veins, which bring the
blood from different parts of the body and discharge it into the vena
cava, by means of which it is conveyed to the heart; (2), the pulmonary
veins, which bring the arterial, or bright red blood from the lungs and
carry it to the left auricle; (3), the veins of the portal system, which
originate in the capillaries of the abdominal organs, then converge into
trunks and enter the liver, to branch off again into divisions and
subdivisions of the minutest character.
The _Capillaries_ form an extremely fine net-work, and are distributed
to every part of the body. They vary in diameter from 1/3500 to 1/2000
of an inch. They are so universally prevalent throughout the skin, that
the puncture of a needle would wound a large number of them. These
vessels receive the blood and bring it into intimate contact with the
tissues, which take from it the principal part of its oxygen and other
elements, and give up to it carbonic acid and the other waste products
resulting from the transformation of the tissues, which are transmitted
through the veins to the heart, and thence by the arteries to the lungs
and various excretory organs.
The blood from the system in general, except the lungs, is poured into
the right auricle by two large veins, called the superior and the
inferior _vena cava_,' and that returning from the lungs is poured into
the left auricle by the _pulmonary veins._
During life the heart contracts rhythmically, the contractions
commencing at the base, in each auricle, and extending towards the apex.
Now it follows, from the anatomical arrangement of this organ, that when
the auricles contract, the blood contained in them is forced through the
auriculo-ventricular openings into the ventricles; the contractions then
extending to the ventricles, in a wave-like manner, the great proportion
of the blood, being prevented from re-entering the auricles by the
tricuspid and mitral valves, is forced onward into the pulmonary artery
from the right ventricle, and into the aorta from the left ventricle.
When the contents of the ventricles are suddenly forced into these great
blood-vessels, a shock is given to the entire mass of fluid which they
contain, and this shock is speedily propagated along their branches,
being known at the wrist as the _pulse_.
On inspection, between the fifth and sixth ribs on the left side of the
chest, a movement is perceptible, and, if the hand be applied, the
impulse may be felt. This is known as the throbbing, or beating of the
heart.
If the ear is placed over the region of the heart, certain sounds are
heard, which recur with great regularity. First is heard a comparatively
long, dull sound, then a short, sharp sound, then a pause, and then the
long, dull sound again. The first sound is caused mainly by the
tricuspid and mitral valves, and the second is the result of sudden
closure of the semilunar valves.
No language can adequately describe the beauty of the circulatory
system. The constant vital flow through the larger vessels, and the
incessant activity of those so minute that they are almost
imperceptible, fully illustrate the perfectness of the mechanism of the
human body, and the wisdom and goodness of Him who is its author.
Experiments have shown that the small arteries may be directly
influenced through the nervous system, which regulates their caliber by
controlling the state of contraction of their muscular walls. The effect
of this influence of the nervous system enables it to control the
circulation over certain areas; and, notwithstanding the force of the
heart and the state of the blood-vessels in general, to materially
modify the circulation in different spots. Blushing, which is simply a
local modification of the circulation, is effected in this way. Some
emotion takes possession of the mind, and the action of the nerves,
which ordinarily keep up a moderate contraction of the muscular coats of
the arteries, is lost, and the vessels relax and become distended with
arterial blood, which is a warm and bright red fluid; thereupon a
burning sensation is felt, and the skin grows red, the degree of the
blush depending upon the intensity of the emotion.
The pallor produced by fright and by extreme anxiety, is purely the
result of a local modification of the circulation, brought about by an
over-stimulation of the nerves which supply the small arteries, causing
them to contract, and to thus cut off more or less completely the supply
of blood.
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