2. Describe blood as it flows from a vein.

3. What is the diameter of the smallest and largest blood cell, and of that of man?

4. Mention the relative size and number of the white and red cells of the blood.

5. Compare arterial and venous blood, noting the circumstances under which the difference may not be apparent.

6. State the circumstances which promote, retard, or prevent the coagulation of the blood.

SENIOR.

1. On what grounds can you advise the free administration of food before capital operations?

2. Sketch the microscopical characters of the red cells under various conditions.

3. What do you know of the development of the blood-cells? 4. Arrange a table of the components of the blood showing their respective proportions in the serum and clot.

5. Adduce proofs that the coagulation of blood depends on the removal of alkali, and suggest some curative deductions.

6. What precautions should be adopted in transfusing blood?

CIRCULATION.

WHEN the nature of the offices which the blood performs is understood, it must be evident that a necessity exists for its distribution to every part of the body. Leaving the left ventricle, it traverses the aorta and every other artery, passes through the capillaries to the veins, and thus ultimately returns to the right auricle. This is the greater or systemic circulation. The blood then passes to the right ventricle, thence through the pulmonary arteries to the lungs, the great organs for purification, after which the pulmonary veins return it to the left auricle. This is the lesser or pulmonary circulation, and is succeeded by the systemic one again. The vena

portæ, as will be explained hereafter, presents the peculiarity of ending in a second set of capillaries, instead of, like other veins, in a trunk.

These facts are now known so generally that it is hard to conceive how slowly each of them was discovered. Physiologists, at first, believed the arteries to be empty, that the cavities of the heart communicated, and the veins carried the blood through the body; but Galen, who lived about A.D. 150, maintained that the arteries carried blood, wrongly guessing that air was taken in for cooling the blood. Vesalius proved there was no communication between the sides of the heart. He suffered punishment and much odium, for having opened the body of a young nobleman dead some hours, the heart being found still beating. Servetus next discovered that the blood flowed from the right side of the heart, through the lungs, to the left; and this pulmonary circulation was re-discovered by Casalpinus, who first applied the term "circulation." It was reserved for our immortal Wm. Harvey to prove the course of the blood through the system, which he did in 1619; but afraid of the scorn of his contemporaries, he delayed publishing his discovery till 1628, when his "Exercitatio Anatomica de motu cordis et sanguinis" was given to the world. He relied on the following proofs :-In bleeding, the vein swells below the ligature, and the blood flows from the side of the orifice farthest from the heart-the contrary being the case with an artery; again, the direction of the valves of the heart and of the veins, which he had seen his master, Fabricius, first demonstrate at Padua. The actual observation of the blood in the capillaries gave further proof; Malpighi made this discovery in 1661-but with Descartes and a few others he had long supported Harvey's doctrine, and defended it from the envious attacks of narrower minds.

The organs concerned in circulation are the heart, arteries, capillaries, and veins, in which order we shall describe their functions.

I. The Heart must be regarded as the most perfect of machines, as it executes with unsleeping activity some three thousand millions of beats, propelling about five hundred thousand tons of blood in the course of an ordinary life. Its descriptive anatomy we shall not attempt, but refer with pleasure to a masterly account in the late lamented Prof. Power's work on the "Heart and Arteries.” It is essentially double in adult life, consisting of one auriele and ventricle on the right side presiding over pulmonary circulation, and two similar cavities on the left performing the systemic circulation. The heart is mainly composed of muscular fibres, very similar to those of voluntary muscles in structure, colour, and tendinous attachment, but peculiar in the following particulars:They are of but the size, according to Skey; their myolemma is so fine as, till lately, to have escaped detection, but it is evident in true fatty degeneration of the heart; they interlace, bifurcate, and anastomose, and do not lie on the same plane, but pursue a spiral direction. In hearts hardened by boiling and alcohol Searle has traced two kinds of ventricular fibres—“ proper," which form two sacs open towards the arterial mouth and at the apex; and "common," which surround both ventricles, and entering through the open ends of the sacs form the inner surface of the ventricles. From the manner in which the fibres mat together, a smooth surface and not a fibrous appearance is obtained by section, as seen well in a roasted sheep heart. To trace the fibres Mr. Ledwich selects an infant's heart, and prepares it with a solution of corrosive sublimate and alum. White fibrous tissue is found round the auriculo-ventricular openings, extending into the active valves round the arterial mouths and their semilunar valves, and scantily in the Eustachian valves in right auricle. The heart is covered by a smooth membrane, the pericardium, to facilitate its own movements, and lined by a fine epithelium, the endocardium, to facilitate the move

ments of the blood. The organ is abundantly supplied by the 2 coronary arteries, whose blood is returned to right auricle by a single coronary vein.

The Nervous Supply is derived from the 3 cervical ganglia of the sympathetic, pneumogastric, recurrent laryngeal, and perhaps phrenic nerves. The nerves form one great, and many minute plexuses, and are finally distributed round the coronary arteries. The pneumogastric appears to be the motor nerve, as its destruction arrests the motion of the heart. The ganglion of Wrisberg lies in the arch of the aorta; and between the ventricles and in the muscular tissue, especially in the hearts of the calf and the frog, Remak has discovered many ganglia. Those described on the surface of the ventricles by Lee, are probably mere thickening of neurilemma.

The Actions of the Heart will be best understood by following the course of the blood, remembering, however, that the 2 auricles and 2 ventricles are synchronous; that is, the contraction or systole of the right ventricle, occurs at the same time as that of the left, and their diastole or dilatation correspond also, and similarly the auricles.

When the right auricle has gradually filled from the cavæ and coronary vein, it suddenly contracts, and the cavæ being filled and the coronary vein being closed by its valve, the blood rushes into the right ventricle. However, some blood is thrown to the large veins, producing the " venous pulse."

The right ventricle, when full, contracts and propels the blood into the pulmonary artery, its passage back into the auricle being prevented by the tricuspid valve. This is closed by the pressure of the blood tending towards auricle, and, as Haller saw in the heart of a living animal, by the shortening of the papillary muscles, which many believe, merely prevent the valves being thrown into auricle. That some reflux was allowed by

the incomplete apposition of the flaps of the tricuspid valve, occurred to Hunter, who says: "The valves in the right side of heart do not so perfectly do their duty as those of the left." This was again stated by Mr. Robert Adams, and still more fully described by Mr. W. King. This anatomist showed, that the anterior or "yielding" wall when thrown forward, pulled with it the anterior and right flaps of the valve, which he calls "curtains of distension," and allowed regurgitation. Any undue separation is prevented by a "moderator band,' which runs from the yielding to the fixed wall or septum. Were it not for this hydraulic imperfection, rupture, which, as it is, never occurs here, would be risked, and over-distension of the cavity would lead to its paralysis, a condition which we shall see occurs in asphyxia. The auricles do not empty themselves as perfectly as the ventricles. The valvular apparatus of the right side and some other structures of the heart, are illustrated by the figure on opposite page.

After aeration the blood proceeds from the lungs through 4 pulmonary veins to left auricle, where no valves exist; and thence into left ventricle, where the mechanically perfect bicuspid valve allows no leakage, and rupture has occasionally occurred at the rete cordis near apex. This powerful cavity gradually contracts, and the blood rushes through aorta and the whole arterial system, to be again returned to the right side of the heart. The aortic valves, like those of the pulmonary artery, offer no obstruction, as they are thrown back against the walls of the vessel when the blood is rushing out; when, however, it falls back, some gets behind them and they are forced together, effectually closing the aperture. The corpora Arantii are of use in keeping the edges of the valves out from the aortic walls, thus letting the blood behind the valves, in strengthening them, and perhaps filling the interval which the 3 valves might leave between them. As Savory has shown, the valves are