The blood passes through different kinds of vessels. Those leading from the heart are called arteries, and those returning to the heart are called veins. As the arteries proceed out from the heart they divide continually, becoming smaller and smaller until they terminate in very small thin-walled vessels called capillaries. These capillaries unite and form veins. Thus the blood is continually flowing from the heart through the arteries and capillaries into the veins and back to the heart.
THE CIRCULATORY SYSTEM.
Notice that the veins (white) are outside of the arteries (black).
As a rule the arteries are below the surface of the body, where they are protected, but if the finger is placed on the wrist or the side of the face near the ear, an artery can be felt through which the blood is pulsing. The veins can be seen in the back of the hand and a pin piercing the body anywhere will break open some of the capillaries and cause blood to ooze out. The capillaries spread throughout the entire tissue of the body and supply with food and oxygen the different living cells of which the body is composed.
The heart is a muscular force pump composed of four chambers, two auricles and two ventricles. It is shaped somewhat like a pear and is situated almost directly behind the breastbone.
The blood coming back from the veins flows into the right auricle, a chamber with rather flabby walls. From here, it passes through a valve into the right ventricle, which is a chamber with very thick muscular walls. From the right ventricle, the blood is driven out through the arteries, capillaries and veins of the lungs, where carbon d i o x i d e i s g i v e n o f f a n d o x y g e n absorbed by the red corpuscles.
R e t u r n i n g f r o m t h e l u n g s , t h e blood enters the left auricle and whenCROSS SECTION OF THE HUMAN HEART.
Showing auricle, ventricle and ventricle valve.
this becomes full, passes through a valve into the left ventricle. This has such powerfully muscular walls that it is able to force the blood throughout the body and back again to the right auricle. As the blood leaves either ventricle, there are valves that close and prevent its return. If the hand is placed a little to the left of the breastbone, the strong contraction of the ventricle can be felt.
108.The Senses. -In order that the brain may communicate with the outside world and so be able to protect the animal from destruction and to provide for its wellbeing, animals have become provided with a number of sense organs which communicate with the brain by the nerves. The most conspicuous sensations of the human body are sight, hearing, taste, smell and touch.
The organ of sight, the eye, is an exceedingly sensitive, automaticallyadjustable camera that records through the nerves. The camera box is the hard bony socket in which it is placed, the eyelid is the shutter, and the iris, the diaphragm. The iris is the membrane in the front of the eye which opens or contracts to let in more or less light. In the center of it is a hole, the pupil.
Back of the shutter, or iris, is a small adjustable lens and beyondthis the sensitive plate, the retina. Between the iris and the front ofCROSS SECTION OF THE HUMAN EYE.
The pupil is the opening between the upper and lower parts of the iris as shown in the figure.
the eye is a watery-like material. the aqueous humor, which keeps the front of the eye extended into its rounded form. Back of the lens is a thick, transparent, jelly- like material, the vitreous humor, which holds the retina extended and keeps the eye from collapsing.
Instead of moving the retina back and forth to focus a picture, asis done with the ground-glass plate in a camera, the eye lens is capable of adjusting itself so as to focus objects which are at different distances.
Leading back to the brain from the retina is the optic nerve, which carries the impressions made on the retina to the brain where they are interpreted into the sensation of sight.
This rough comparison is by no
means a deion of the eye, for it is a most complex and wonderful organ, vastly superior in construction to a camera. A technical deion would,however, be out of place here.
CROSS SECTION OF THE HUMAN EAR.
The ear, which is the sound transmitter, consists of the outer ear, which is so arranged as to catch the sound waves and converge them upon the ear drum. The ear drum is a thin membrane stretched tightly across a bony opening and vibrating when the air waves strike it, as a drum does when struck by the drumstick. On its inner side the drum is attached to the inner ear by a chain of three bones. The sensitive cells of the inner ear transmit the impressions made by the sound vibrations through the auditory nerve to the brain, where they are interpreted into the sensation of sound.
On the tongue and in the nose are cells which transmit to the brainthe impressions produced upon them by different qualities, the one of solutions and the other of gases. The sensations thus produced are called taste and smell.
The sensation of touch originates in the skin and is much moreacute in some portions than in others. The tips of the fingers in the blind are often trained to such delicate perception that they, in a great degree, take the place of the lacking sense organ. These sensations, like all others, are carried to the brain by the nerves and there interpreted into the sensation of touch.