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the lungs only. The heart, to carry on a double circulation, requires four cavities, into which its interior is divided by partitions,--an

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1 Right Auricle. 2. Right Ventricle. 3. Left Auricle. 4. Left Ventricle

The blood vessels with the light oross-markings are systemic arteries, and may be coloured red The great systemic artery, the aorta, arises from cavity No. 4, concealed by the pulmonary artery. From the convexity of the arch which it forms, are given off, first, the brachiocephalic, or right arm and head, artery ; second, the left carotid to the left side of the head; and third, the left subclavian for the left arm. The corresponding veins unite to form the vena cava superior. The aorta now turns down in front of the spine, and divides on the

auriclel and ventricle> on each side, right and left, the auricle being merely the ante-chamber to its ventricle, while the ventricle is the strong cavity for filling the arteries.3 The right ventricle sends its blood by the pulmonary artery to the lungs, the left by the aorta to all parts of the body.

The course of the blood then is as follows :- Beginning at the right auricle, No. 1, this cavity receives the blood from the great systemic veins. The right auricle then contracts and fills cavity No. 2, the right ventricle, which in its turn contracts and sends the blood along the pulmonary artery, and its divisions, to the lungs. Purified in the capillaries4 or hair-like vessels of the lungs by the influence of the air which we inhale, and changed from dark or venous, to red or arterial blood, it now returns by the pulmonary veins to cavity No. 3, the left auricle, by which it is sent into cavity No. 4, the left ventricle ; which, finally, contracting powerfully, discharges it into the aorta. From this great systemic artery the blood is distributed at last to the systemic capillaries, blood-vessels which, in minute ramifications, pervade every part of the body. Through the walls of these capillaries it deposits new matter in the tissues of the body, and in consequence of this, becomes changed from red or arterial to dark or venous blood. Having served this purpose, it now passes onwards to the veins, by which it is collected, and at last brought by the two great systemic veins, the vena cava superior, and inferior, to the right auricle, where we began to trace it.

The action of the ventricles would be of little use in propelling the blood if valves were not provided to prevent regurgitation. Accordingly the inlet and outlet of each ventricle is guarded by

fourth lumbar vertebra into the common iliac arteries; these divide into the internal and external iliacs, the latter being the artery for the inferior extremity.

The pulmonary artery, arising from No. 2, is marked with dotted crossed lines.

The clear vessels are the veins. Those connected with No. 1 of the heart may be coloured blue. Those joining No. 3. are the pulmonary veins, and are seen, two in number, at the lower part of the root of each lung. The pulmonary veins may be coloured red.

The dark tube with strong cross-markings leading to each lung, is the windpipe, surmounted by the larynx.

The liver is seen to receive a large vein, the vena portæ, in addition to its artery wbich is smal. The returning veins, or veins proper, of the liver are seen entering the vena cava inferior.

i Auricula, a little ear, from a fancied resemblance. 2 Ventriculus, a small belly, or cavity.

8 Arteria, from aer, air, and tereo, to pierce through; from the ancient notion that the arteries conveyed air, because they are mostly found empty after death.

4 Capillary, from Capillus, a bair of the head.

Fig. 14.

valves. The valve at the inlet is termed the auriculo-ventricular, right and left, or tricuspidl valve on the right side, and mitral valve on the left. It is composed of an apparatus of membranes, cords, and muscles, like the sails of a boat with their ropes. When the blood tends to regurgitate, the membranes are floated back, against each other, the cords prevent them being washed through, while the muscles shorten the cords, like sailors holding on by and tightening the ropes of a sail.

The outlet of each ventricle is guarded by the three semi-lunar or halfmoon-like valves. The whole body is full of evidences of design, but perhaps no part presents a contrivance at once more simple and beautiful than this. These valves, shown in Figure 14, may be com

pared to three swallow's nests built
in the mouth of a pipe, but made
of soft silk-like membrane, so as to
be easily pushed aside and down
again. The part of the wall at
and above the cavity of the nest
is excavated, or bulged out, form-
ing the so-called sinus or hollow of
Valsalva. The blood having in its
passage upwards, driven the valves
aside, now tends to regurgitate, but
getting behind the valves, which
is made quite sure of by the exist-
ence of the sinus,-throws them
down and against each other, so
that the aperture is completely
blocked up. This action is perfectly

mechanical, as may be proved by Aorta slit open, showing the three semi- experiment on the dead animal. lunar valves. The dark spaces above each The same principle is carried out

in the veins and lymphatic vessels whenever a valve or pair of valves exists, each being provided with a sinus, or bulging, behind it, to make quite sure of the valve being thrown down.3

1 Tricuspid, three-pointed.
2 Mitral, from a fancied resemblance to a mitre ; also termed bicuspid.

These, and the other valves of the heart may be demonstrated from the heart of a sheep, or other quadruped. The semi-lunar valves are seen on slitting up the aorta and pulmonary artery : the tricuspid and mitral valves, on opening the ventricles.

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valve are the sinuses of Valsalva.

The making of these semi-lunar valves would seem

an easy matter compared with their preservation. When we recollect that, at every pulse of the heart they are dashed up, and then dashed down again with such force as to cause a noise which may be heard externally, and is known as the second or sharp sound of the heart ; that they bear a strain equal we may safely say to a pound weight, until again dashed up by the next pulse ; that this takes place seventy-five times, more or less, in a minute, that is, 4500 times every hour ; that their texture is thin, and that their giving way would be fatal ; and when we consider how short a time any such contrivance of ours would stand so much rubbing, -we can understand how wonderfully we are made, and how far the wisdom and power of God passeth that of man.

NUTRITION-SECRETION-RESPIRATION.

1. The immediate source of nourishment is the blood. The process of Nutrition' or assimilation, takes place through the walls of the capillaries, which so closely pervade almost every part of the body that a fine needle cannot be introduced without drawing blood, and is, indeed, a very coarse instrument among them. The change is a double one, new matter being laid down and waste matter taken up, the circulation performing at once the double function of the baker's and the scavenger's cart. The process is the result of a vital power given to each tissueby which it is capable of reproducing its like from the common supply; and likewise in the case of different animals, as the dog and cat, each builds up its peculiar framework, it may be, from the very same kind of food. It is so ordered that like shall produce its like, whether in the case of texture, individual, or species.

2. Waste matters are cast out of the body by several secreting, or excreting, organs : the lungs, liver, kidneys, and skin. Respiration, or breathing, is the most important of these means, removing carbonic acid, or poisonous gas from the blood, besides throwing off watery vapour, and introducing oxygen, or pure gas, into the blood in its stead. The general effect of the animal and vegetable kingdoms on the atmosphere is opposite; the animal removes oxygen and supplies carbonic acid ; the vegetable consumes carbonic acid and sets free the oxygen ; these two great kingdoms of nature thus balancing and supplying each other. The breathing of a number of persons, therefore, gradually poisons the air of a room, by impregnating it with carbonic acid, causing first warmth, and then drowsiness ; and, were all the doors and windows quite closed, death would gradually ensue. Hence the necessity for the ventilation of our houses, schoolrooms, workshops, and places of public assembly. No law of health is more neglected than this one, as if it were an object to shut out the light and the fresh air of heaven. This neglect, aided by insufficient clothing and nourishment, is one of the most fruitful causes of ill health and disease, especially of consumption.

· Nutrio, to nourish.

2 Tissue, or ture, the term applied to the elementary structure of which any organ is composed. Thus we speak of muscular tissue, osseous tissue, nervous tissue, &c. A mass of muscular tissue forms a muscle, of bony tissue, a bone, and so on. The peculiar elementary character of the tissue is seen by examination with the microscope. Thus ligament, or tendon, is composed of fine fibres or threads ; nerves, of fine tubes; fat, and grey nervous matter, of minute cells. When several simple or elementary textures are woven together, as in the skin and other membranes, we speak of these parts as compound tissues. Each tissue has its own vital properties.

3. The mechanical part of respiration is accomplished by the chest's moving like a pair of bellows, but without the hole below, the air being drawn in, as well as pushed out, at the nozzle ; while the muscles attached to the ribs do for the chest, in respiration, what those of the hand and arm do for the bellows. At the upper part of the windpipe, is placed the Larynx, in the interior of which is the glottis, a triangular narrowed part, left between the vocal cords, which, by enlarging and contracting, and by the varied tension of the cords, acts both as a valve or check on the passage, and as the producer of vocal sound. Articulation, or speech, is effected by the parts above, as the lips, teeth, tongue, palate, and nose ; and is, like learning to play on any other instrument, an artificial acquirement. Hearing is necessary to it, and hence those born deaf must remain dumb; rational intelligence is no less indispensable, and hence the imbecile and the lower animals cannot be taught to speak.

The windpipe proper, or Trachea (Fig. 13), divides for each lung, and ramifies like the branches and twigs of a tree, ending at last in little rounded air cells. The lung is composed of an immense number of these little air cells. The pulmonic capillaries form a very close network on and in the delicate wall membrane of each cell, and the exchange of gases takes place through the membranes, the pure air passing into the blood, and the impure gas passing out from the blood into the air cell. The carbonic acid or impure gas set free is not formed here, but in the capillaries

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