tem, and of thus preventing the right heart from being overcharged with venous blood during the temporary suspension of respiration. By means of these arterial reservoirs, the cetacea can support life under water for a quarter of an hour, or even longer.

Another peculiarity deserving of notice is, that occasionally a large artery will divide into a great number of smaller vessels, which again reunite to form a single trunk. An arrangement of this kind is known as a rete mirabile, and a good exam ple of it occurs within the skull in long-necked grazing animals, the object being to check too strong a current of blood to the brain.

In birds, the heart is usually of a very large size, as compared with the bulk of the body. The trunk of the aorta is extremely short, and divides into three main branches, the central one forming the descending aorta, while the two lateral ones give off the carotid and subclavian arteries on either side. The branches of the fatter give an abundant supply of blood to the powerful thoracic muscles by which the wings are moved.

In the class of reptiles, there is not a complete double C., a mixture of arterial and venous blood being sent both to the lungs and to the general system. In fig. 6, the general nature of the C. in this class is typically represented. The heart consists of two auricles and one ventrical. The impure blood which has circulated through the system is conveyed by the vena cava into the right auricle, from whence it passes into the common ventricle. At the same time, blood which has been aërated in the lungs is poured into it from the left auricle; hence the ventricle contains an admixture of venous and arterial blood. As both a pulmonary artery and an aorta are given off by the ventricle, the latter by its contractions simultaneously drives one portion of its contents to the lungs, and another to the general system. In this way, a semi-oxygenated blood is transmitted to the various parts of the body, the only pure blood being that which is contained in the left auricle, and in the veins opening

into it.

Although the above may be regarded as the general type of the circulating apparatus in reptiles, yet there are many modifications of it (into which we have not space to enter), which connect it on the one hand (in the case of the Perennibranchiate amphibia, such as the axolotl, proteus, &c.) with that of fishes, and on the other hand (when there is a more or less perfect separation of the ventricular cavity, as in the crocodiles) with that of birds and mammals.

Fig. 6.-Circulation in Rep

tiles;

h, heart, enclosed in pericardium; f, f', right and left auricles; v, single ventricle; a, aorta; b, vena cava; c, smaller circula-/ tion; d, greater circulat'n.

In the class of fishes, the circulating apparatus is far simpler than in reptiles. The heart possesses only two cavities, an auricle and a ventricle, and is traversed solely by venous blood; hence it is analogous to the right side of the mammalian heart. Venous blood is brought by veins, which correspond with our vence cave, from all parts of the system, and enters the auricle (see fig. 7); from the auricle, the blood passes into the ventricle, which is of great muscular strength; and the ventricle propels its contents through a vessel which corresponds with our pulmonary artery, and which dividing on either side into four or five branches, goes to the gills, in the capillaries of which it becomes oxygenated, by means of the air that is diffused through the water. From the filaments and fringe-like structures of the gills, it is at length collected into a large trunk, commonly called the dorsa sal vessel, but analogous to the aorta of mammals and birds, in so much as it sup plies the whole body with arterialised blood. After passing through the systemic ti capillaries, the blood returns in a venous condition to the heart, and the above pro-a cess is repeated. Although the heart is simpler than in reptiles, the C. is in one sense of a higher character, in so far as pure arterial (not mixed) blood is here

conveyed to all parts of the system; hence, probably, the far greater muscular energy of fishes may be explained.

We can only allude very briefly to the C. in the invertebrate animals.

In the mollusca, we find hearts of varying complexity, usually with one or two auricles, and one ventricle; but in all cases, the auricle or auricles receive aerated blood from the respiratory organs, and pass it to the strongly muscular ventricle, which propels it over the body. The heart is therefore a systemic heart. There seem to be no capillaries in these animals, excepting in the respiratory organs; the blood leaving the open ends of the arteries, passes into the interstices (lacunae) of the parenchyma of the body, from whence it is taken up by the open mouths of the venous radicles; hence this kind of C. is called lacunary.

In the crustacea, the form of the heart and the number of its orifices presents several modifications; the following is, however, the typical mode of C. of these animals. The heart, which is here a single cavity, is sometimes round, and sometimes long and tubular, and is the point of departure of the arterial system, which

In some of the ascidians and in salpa, the following remarkable phenomenon occurs: The heart, which is extremely simple, and of course without valves, at definite intervals (of about twenty minutes) reverses the direction of its current. Before the heart changes the direction of its contractions, it remains still for a short time, and the blood-currents in the body are thus slackened in their course before they receive an impulse in the opposite direction. The vessels entering and. leaving the heart thus act alternately as an aorta and as a vena cava..

Circumcision

consists of trunks emerging in various directions. The blood returning from the arteries does not enter into distinct veins, but into irregular excavations in the tissues, which are termed venous sinuses; from these venous sinuses it passes to the gills, from whence it is returned to the heart in an aërated state by the branchiocardiac canals; so that here, as in the mollusca, the heart is systemic.

It is unnecessary for us to notice the comparatively imperfect C. in insects and animals lower in the scale than those we have already considered.

We now approach the last part of the subject-the physiology of the circulation.

We shall consider-1. The flow of blood through the heart; 2. The phenomena of the arterial C.; 3. The phenomena of the capillary C.; and, 4. The phenomena of the venous circulation.

1. Direct observation and experiment clearly shew, that the muscular contraction of the heart is the principal source of the power by which the blood is propelled in its course. This action of the heart may be observed by opening the chest of a living animal or, better still, of an animal deprived of sensation and motion by poison, and in which artificial respiration is kept up. It is then seen to consist of two motions-first, a contraction or systole of the auricles, and second, a corresponding contraction of the ventricles. The contraction of the auricle immediately precedes that of the ventricle, and the systole of each cavity is directly followed by its diastole or relaxation; there is then a brief period of repose, the heart exhibiting little or no motion. At the moment of the systole of the ventricles, the apex of the heart is tilted forwards, causing a pulsation against the ribs that can be felt externally.

The force exerted by the left ventricle has been so very variously estimated, that we must regard this point as still unsettled. The number of contractions of the heart of an adult in a minute is about 70 or 75; it is, however, liable to great variations, which will be noticed in the article PULSE. The sounds accompanying the heart's action, which may be readily heard by applying the ear either directly or through the medium of the stethoscope to the cardiac region, are discussed in the article HEART, Sounds of THE.

2. The arteries exercise a vast influence on the movement of the blood through them, in virtue of two properties which they possess-viz., elasticity and contractility. These two endowments are not equally and uniformly possessed by the whole arterial system-elasticity (the property by which the interrupted or discontinuous force of the heart is made equable and continuous) existing chiefly in the larger trunks; while contractility-which is more required for regulating the flow of blood to particular parts-is most marked in the smaller vessels. The rate of movement of the blood through the arteries in man can only be roughly calculated from experiments on animals. Volkmann finds that in the carotids of mammals, the average velocity of the blood stream is about 12 inches per second; he has likewise ascertained that the velocity is greater in arteries lying near than in those at a distance from the heart, that it is not increased by an augmentation in the number of pulsations, but that it is greatly augmented by an increase in the volume of the blood, and lessened by its diminution.

3. It has long been a debated point whether the capillary C. is influenced by any other agency than the contractility of the heart and arteries. Harvey believed that the action of the heart alone was sufficient to send the blood through the whole circuit, and in recent times his view has been supported by J. Müller and other eminent physiologists. On the other hand, Professor Draper of New York holds the opposite extreme view, asserting that "it is now on all hands conceded that the heart discharges a very subsidiary duty." We believe that Bichat was the first to maintain the opinion that the capillaries are organs of propulsion, and are alone concerned in returning the blood to the heart through the veins. Although Bichat attributed too great power to the capillaries, there cannot be a doubt that the movement of the blood through these vessels is not solely due to the heart; in short that there is what may be termed a capillary power. The following are a few of the facts proving this to be the case: 1. On watching the C. in the web of a frog's foot, it is at first seen to go on with perfect regularity. After a time, however, various changes are observed, which cannot be attributed to the heart, such as alterations in the size of some capillaries, and in the velocities of the currents passing through

them, and occasionally even a reversal in the direction of some of the lesser currents. 2. In cool-blooded animals, the movement of the blood in the capillaries continues long after the excision of the heart. 3. Actual processes of secretion not unfrequently continue after death; sweat, for instance, may be exuded from the skin, and other secretions may be formed by their respective glands, which could not take place if the capillary C. had stopped. 4. Cases occasionally occur in which a fœtus without a heart is produced, and yet in these cases most of the organs are well developed.

What the nature of this capillary power is, is not clearly known. Professor Draper and others have endeavored to explain it on the principals of capillary attraction. There is no satisfactory evidence that the capillaries possess true contractility, for, although their diameter is subject to great variations, this may be due simply to the elasticity of their walls. If we could only establish their contractility, the difficulty would be removed.

The rate of movement of the blood through the capillaries is about 1.2 inch per minute in the systemic capillaries of the frog. In the warm-blooded animals it is probably more rapid. From Volkmann's observations, the rate in the dog is about 1-8 inch per minute.

4. It is usually estimated that the venous system contains from two to three times as much blood as the arterial. The latter is probably the more correct ratio, and as the rapidity of blood in the two systems seems to bear an inverse ratio to their respective capacities, the venous blood will move with only one-third of the velocity of arterial blood. We have already noticed the occurrence of valves in the venous circulation. Their object is evidently to prevent the reflux of blood; hence they are of important use in the maintenance of this part of the circulation. They are most abundant where their is much muscular movement. The movement of blood through the veins is undoubtedly mainly due to the visa tergo resulting from the contraction of the heart and arteries. This is much assisted in many parts of the system by the constantly recurring pressure of the adjacent muscles upon their trunks. The movement of inspiration, by causing a comparative vacuum in the chest, has been supposed by some physiologists to assist the flow of venous blood to the heart, and a similar influence has been ascribed to an assumed suction-power of the heart. The contractility of the veins in man is too slight to produce any marked effect on the propulsion of the current. From the investigations of Professor Wharton Jones "on the rhythmical contractility of the veins of the bats's wing," we may infer that, in many of the lower animals, it is probably a more efficient power. In connection with this article, consult ARTERY, ČAPILLARIES, PULSE, and VEIN.

CIRCULATION OF SAP in plants-its ascent from the root to the leaves and bark, and its partial descent after the elaboration which it undergoes in these organs. The sap drawn from the ground by the roots (see OSMOSE) ascends in exogenous plants, which have hitherto been principally the subjects of examination, through the more recent parts of the woody tissue, and especially through the alburnum. The descent of the sap takes place chiefly through the liber or inner bark. It appears certain also that, on its return to the root, only a small portion is excreted, and that the greater part ascends again, readapted to the use of the plant by the excretion which has taken place. Much of the sap which is taken up by the roots is, however, thrown off in perspiration by the bark and leaves. The sap is also laterally diffused through the cellular tissue of plants, and very interesting observations have been made by Schultz and others on peculiar movements of the elaborated or descending sap (latex). Many physiologists dislike the term circulation applied to sap, as suggesting a closer analogy than really exists to the circulation of the blood in animals. See PLANT, LEAVES, and SAP.

CIRCUMCISION (Lat. a cutting around), the cutting off the foreskin (præputium), a rite widely diffused among ancient and modern nations. The prevalent idea among Christians was (and perhaps still is), that the rite originated with Abranam, who (as we read in Gen. xvii. 9-14) was commanded by God to circumcise himself and his whole household, and to transmit the custom to his descendants. But, as Jahn ("Biblische Archäologie," Vienna, 1797-1800) acutely observes, this is inconsistent with the very terms in which the command is expressed, these terms re-supposing a knowledge of the rite on the part of Abraham. That it existed

Circus

previously to the time of the patriarch, however, seems to be indisputable. The researches of modern scholars prove that the Egyptians, for instance, were in the habit of circumcising long before Abraham was born. Rawlinson, in a note to his version of Herodotus, remarks that "circumcision was already common in Egypt at least as early as the fourth dynasty of kings, and probably earlier, long before the birth of Abraham, or 1996 B.C." The testimony borne by the monuments of Upper and Lower Egypt (consult Sir Gardiner Wilkinson's "Manners and Customs of the Ancient Egyptians") is to the same effect, and apparently conclusive. Another argument which has been adduced against its Abrahamic origin, is the fact of its being so extensively practised. At the present day, it may be traced almost in an unbroken line from China to the Cape of Good Hope. It is also a usage in many of the South Sea Islands, and the followers of Columbus were much astonished to find it existing in the West Indies, and in Mexico. Recently, too, it has been ascertained to have been long practised by several tribes in South America. Such being the case, many scholars hold it impossible to suppose that the origin of so universal a rite can be traced to a single Semitic nation, more especially when that nation was peculiarly averse to intercourse with other nations, and in other respects exercised no overt influence on their customs. Whether, as Jahn supposes, Abraham obtained his knowledge of C. from the Egyptians, we cannot determine. It would appear, however, that the Canaanites among whom he came to reside were not circumcised, for we read of the Prince of Shechem and his people undergoing the operation, that the former might obtain the hand of Dinah, daughter of Jacob; and the institution of it in the family of Abraham was probably sufficient to mark off that family from the surrounding tribes. In the case of Abraham and his descendants, the rite acquired a religious significance. It was ordained to be the token or seal of the everlasting covenant between God and his people. Such is the view of St Paul, who looked upon the C. of the foreskin as symbolical of the C. of the heart; and that along with all that was merely Judaistic and material, it was abrogated by the more spiritual teaching of Christ.

The time for C. among the Jews is the 8th day after the birth of the child; among the Arabians the 13th year, in remembrance, it is said, of their ancestor Ishmael; among the Kaffirs, at a still later period, marking, in fact, the transition from youth to manhood; and, indeed, each nation seems to have selected the time most agreeable to its own notions of what is prudent or becoming. The Abyssinians are the only people professing Christianity among whom C. is practised. The C. of females, or what is equivalent to such, is not unknown among various African nations. For fuller information in regard to C. consult Sonnini's "Travels in Egypt," Sir John Marsham's "Chronicus Canon Ægyptiacus," and Winer's "Biblisches Realwörterbuch."

CIRCUMFERENCE, or Periphery, the curve which encloses a plane figure: thus, we speak of the circumference of a circle, or of an ellipse; but in figures bounded by straight lines, as the triangle, square, and polygon, the term perimeter is employed to designate the whole bounding lines taken together.

CIRCUMNAVIGATION, the term usually applied to the act of sailing round the world, its literal meaning being simply a sailing round. The C. of the globe was at one time considered a great feat, but it is now regarded as one of the most commonplace affairs in a sailor's experience. The first to circumnavigate the globe was Magalhaens (q. v.), or Magellan, a Portuguese, in 1519; eighteen years afterwards, it was accomplished by a Spaniard; and in 1577 by the illustrious Englishman, Drake. The most celebrated of circumnavigators, however, was Captain James Cook, who, between 1768 and 1779, made three voyages round the world. CIRUMSTANTIAL EVIDENCE. See EVIDENCE.

CIRCUMVALLA'TION, in Fortification, is a series of works surrounding a place when under siege; not to serve offensively against the place, but to defend the siege. army from an attack from without. It usually consist of a chain of redoubts, either isolated or connected by a line of parapet. Such lines were much used in the sieges of the ancient and middle-ages; but in modern times they are not so necessary, because the use of artillery lessens the duration of a siege, and also because the besiegers have generally a corps of observation in the open field, ready to repel any force of the enemy about to succor the besieged. A remarkable example of C. was