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have their origin in the venous radicles, and, as they approach the heart, converge to form larger trunks, and terminate finally in the venæ cavæ.

They possess three coats-
1. External, made up of areolar tissue.

2. Middle, composed of non-striated muscular fibers, yellow, elastic, and fibrous tissue.

3. Internal, an endothelial membrane, similar to that of the arteries.

Veins are distinguished by the possession of valves throughout their course, which are arranged in pairs, and formed by a reflection of the internal coat, strengthened by fibrous tissues; they always look toward the heart, and when closed prevent a return of blood in the veins. Valves are most numerous in the veins of the extremities, but are entirely absent in many others.

The Onward Flow of Blood in the veins is mainly due to the action of the heart, but is assisted by the contraction of the voluntary muscles and the force of respiration.

Muscular contraction, which is intermittent, aids the flow of blood in the veins by compressing them. As regurgitation is prevented by the closure of the valves, the blood is forced onward toward the heart.

Rhythmical movements of veins have been observed in some of the lower animals, aiding the onward current of blood.

During the movement of inspiration the thorax is enlarged in all its diameters, and the pressure on its contents at once diminishes. Under these circumstances a suction force is exerted upon the great venous trunks, which causes the blood to flow with increased rapidity and volume toward the heart.

Venous Pressure. As the force of the heart is nearly expended in driving the blood through the capillaries, the pressure in the venous system is not very marked, not amounting in the jugular vein of a dog to more than 12 that of the carotid artery.

The time required for a complete circulation of the blood throughout the vascular system has been estimated to be from 20 to 30 seconds, while for the entire mass of blood to pass through the heart 58 pulsations would be required, occupying 48 seconds.

The Forces keeping the blood in circulation are-
I. Action of the heart.
2. Elasticity of the arteries.
3. Capillary force.
4. Contraction of the voluntary muscles upon the veins.
5. Respiratory movements.

RESPIRATION. Respiration is the function by which oxygen is absorbed into the blood and carbonic acid exhaled. The appropriation of the oxygen and the evolution of carbonic acid takes place in the tissues as a part of the general nutritive process, the blood and respiratory apparatus constituting the media by means of which the interchange of gases is accomplished.

The Respiratory Apparatus consists of the larynx, trachea, and lungs.

The Larynx is composed of firm cartilages, united together by liga. ments and muscles ; running antero-posteriorly across the upper opening are four ligamentous bands, the two superior, or false vocal cords, and the two inferior, or true vocal cords, formed by folds of the mucous membrane. They are attached anteriorly to the thyroid cartilages and posteriorly to the arytenoid cartilages, and are capable of being separated by the contraction of the posterior crico-arytenoid muscles, so as to admit the passage of air into and from the lungs.

The Trachea is a tube from four to five inches in length, three quarters of an inch in diameter, extending from the cricoid cartilage of the larynx to the third dorsal vertebra, where it divides into the right and left bronchi. It is composed of a series of cartilaginous rings, which extend about twothirds around its circumference, the posterior third being occupied by fibrous tissue and non-striated muscular fibers, which are capable of diminishing its caliber.

The trachea is covered externally by a tough, fibro-elastic membrane, and internally by mucous membrane, lined by columnar ciliated epithelial cells. The cilia are always waving from within outward. When the two bronchi enter the lungs they divide and subdivide into numerous and smaller branches, which penetrate the lung in every direction until they finally terminate in the pulmonary lobules.

As the bronchial tubes become smaller their walls become thinner; the cartilaginous rings disappear, but are replaced by irregular angular plates of cartilage; when the tube becomes less than the so of an inch in diameter they wholly disappear, and the fibrous and mucous coats blend together, forming a delicate elastic membrane, with circular muscular fibers.

The Lungs occupy the cavity of the thorax, are conical in shape, of a pink color and a spongy texture. They are composed of a great number of distinct lobules, the pulmonary lobules, connected together by interlobular connective tissue. These lobules vary in size, are of an oblong shape, and

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FIG. 12.

are composed of the ultimate ramifications of the bronchial tubes, within which are contained the air vesicles or cells. The walls of the air vesicles, exceedingly thin and delicate, are lined internally by a layer of tessellated epithelium, externally covered by elastic fibers, which give the lungs their elasticity and distensibility.

The Venous Blood is distributed to the lungs for aeration by the pulmonary artery, the terminal branches of which form a rich plexus of capillary vessels surrounding the air cells; the air and blood are thus brought into intimate relationship, being separated only by the delicate walls of the air cells and capillaries.

The thoracic cavity in which the respiratory organs are lodged is of a conical shape, having its apex directed upward, its base downward. Its framework is formed posteriorly by the spinal column, anteriorly by the sternum, and laterally by the ribs and costal cartilages. Between and over the ribs lie muscles, fascia and skin; above the thorax is completely closed by the structures passing into it and by the cervical fascia, and skin ; below it is closed by the diaphragm. It is therefore an air-tight cavity.

The Pleura.-Each lung is surrounded by a closed serous membrane, the pleura, one layer of which, the visceral, is reflected over the lung, the

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larynx is seen to branch into two other, the parietal, reflected over the large bronchi, which subdivide after

they enter their respective lungs. wall of the thorax; between the two layers is a small amount of fluid which prevents friction during the play of the lungs in respiration.

Owing to the elastic tissue which is present in the lungs, they are very readily distensible, so much so, indeed, that the pressure of the air inside the trachea and lungs is sufficient to distend them until they completely fill all parts of the thoracic cavity not occupied by the heart and great vessels. The elastic tissue endows them not only with distensibility, but also with the power of elastic recoil, by which they are enabled to accommodate themselves to all variations in the size of the thoracic cavity.

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DIAGRAM

OF THE RESPIRATORY

ORGANS.

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