In the capillary current the red corpuscles may be seen hurrying down the centre of the stream, while the white corpuscles in the still layer adhere to the walls of the vessel, and at times can be seen to pass through the walls of the vessel by amoeboid movements.

The passage of the blood through the capillaries is mainly due to the force of the ventricular systole and the elasticity of the arteries; but it is probably also aided by a power resident in the capillaries themselves, the result of a vital relation between the blood and the tissues.

The Veins are the vessels which return the blood to the heart; they have their origin in the venous radicles, and as they approach the heart, converge to form larger trunks, and terminate finally in the vena cavæ. They possess three coats

1. External, made up of areolar tissue.

2. Middle, composed of non-striated muscular fibres, 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 tissue; 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 aspiration.

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 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 ligaments 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 fibres which are capable of diminishing its calibre.

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 of an inch in diameter they wholly disappear, and the fibrous and mucous coats blend together, forming a delicate, elastic membrane, with circular muscular fibres.

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 inter-lobular connective tissue. These lobules vary in size, are of an oblong shape, and 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 fibres, which give the lungs their elasticity and distensibility.

FIG 7.

Diagram of the respiratory organs. The windpipe leading down from the larynx is seen to branch into two large bronchi, which subdivide after they enter their respective lungs.

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

The lungs are nourished by blood from the bronchial arteries ramifying in the walls of the bronchial tubes and interlobular connective tissue.

Respiratory movements. The movements of respiration are two, and consist of an alternate dilatation and contraction of the chest, known as inspiration and expiration.

1. Inspiration is an active process, the result of the expansion of the thorax, whereby air is introduced into the lungs.

2. Expiration is a partially passive process, the result of the recoil of the elastic walls of the thorax, and the recoil of the elastic tissue of the lungs, whereby the carbonic acid is expelled.

In Inspiration the chest is enlarged by an increase in all its diameters, viz. :

1. The vertical is increased by the contraction and descent of the diaphragm when it approximates a straight line.

2. The antero-posterior and transverse diameters are increased by the elevation and rotation of the ribs upon their axes.

In ordinary tranquil inspiration the muscles which elevate the ribs and thrust the sternum forward, and so increase the diameters of the chest, are the external intercostals, running from above downward and forward, the sternal portion of the internal intercostals and the levatores costarum.

In the extraordinary efforts of inspiration certain auxiliary muscles are brought into play, viz.; the sterno-mastoid, pectorales, serratus magnus, which increase the capacity of the thorax to its utmost limit.

In Expiration the diameters of the chest are all diminished, viz. : 1. The vertical, by the ascent of the diaphragm.

2. The antero-posterior, by a depression of the ribs and sternum. In ordinary tranquil expiration the diameters of the thorax are diminished by the recoil of the elastic tissue of the lungs and the ribs; but in forcible expiration the muscles which depress the ribs and sternum, and thus further diminish the diameter of the chest, are the internal intercostals, the infracostals, and the triangularis sterni.

In the extraordinary efforts of expiration certain auxiliary muscles are brought into play, viz.: the abdominal and sacro-lumbalis muscles, which diminish the capacity of the thorax to its utmost limit.

Expiration is aided by the recoil of the elastic tissue of the lungs and ribs and the pressure of the air.

Movements of the Glottis. At each inspiration the rima glottidis is dilated by a separation of the vocal cords, produced by the contraction of the crico arytenoid muscles, so as to freely admit the passage of air into the lungs; in expiration they fall passively together, but do not interfere with the exit of the air from the chest.

Nervous Mechanism of Respiration. The movements of respira tion are involuntary and reflex, and are under the control of the medulla oblongata.

This centre may be stimulated

1. Directly, by the condition of the blood. An increase of carbonic acid or a diminution of oxygen in the blood causes an acceleration of the respiratory movements; the reverse of these conditions causes a diminution of the respiratory movements.

2. Indirectly, by reflex action. The medulla may be excited to action through the pneumogastric nerve, by the presence of carbonic acid in the lungs irritating its terminal filaments; through the fifth nerve, by irritation of the terminal branches; and through the nerves of general sensibility. In either case this centre reflects motor impulses to the respiratory muscles through the phrenic, intercostals, inferior laryngeal and other nerves.

Types of Respiration. The abdominal type is most marked in young children, irrespective of sex; the respiratory movements being effected by the diaphragm and abdominal muscles.

In the superior costal type, exhibited by the adult female, the respiratory movements are more marked in the upper part of the chest, from the Ist to the 7th ribs, permitting the uterus to ascend in the abdomen during pregnancy without interfering with respiration.

In the inferior costal type, manifested by the male, the movements are largely produced by the muscles of the lower portion of the chest, from the 7th rib downward, assisted by the diaphragm.

The respiratory movements vary according to age, sleep and exercise, being most frequent in early life, but averaging 20 per minute in adult life. They are diminished by sleep and increased by exercise. There are about four pulsations of the heart to each respiratory act.

During inspiration two sounds are produced; the one, heard in the thorax, in the trachea and larger bronchial tubes, is tubular in character; the other, heard in the substance of the lungs, is vesicular in character.

AMOUNT OF AIR EXCHANGED IN RESPIRATION, AND CAPACITY OF LUNGS.

The Tidal or breathing volume of air, that which passes in and out of the lungs at each inspiration and expiration, is estimated at from 20 to 30 cubic inches.

The Complemental air is that amount which can be taken into the lungs by a forced inspiration, in addition to the ordinary tidal volume, and amounts to about 110 cubic inches.