9 cles. In the ornithorhynchus, or duck-billed platypus, the heart, in some respects, resembles that of birds, We likewise find certain varieties in the distribution of the blood-vessels. Thus, while in man the subclavian and carotid arteries arise on the right side from a short common trunk given off by the aorta, and on the left side arise directly from the aorta, we find several varieties of this arrangement in the mammalia. In the horse and the ruminants, the aorta divides at once at its origin into an anterior trunk, which gives off the carotid and subclavian arteries of both sides, and a posterior trunk for the thoracic and abdominal aorta. In the dolphin, and in some-if not allof the bats, two short trunks (arteria innominata) arise, and give off each a carotid and subclavian on either side. In the elephant, both carotids are given off from a single common trunk, situated midway between the two subclavians. All these, and other varieties which might be noticed, are occasionally found in man; and it may be laid down as a general rule, that when any abnormal arterial distribution is detected in the human subject, it represents the normal type in some lower mammal. A very remarkable peculiarity in the distribution of the vascular system (both arteries and veins) is exhibited by the cetacea and other diving animals, in which the respiration, and consequently the arterialization of the blood, is temporarily stopped. Various arteries of the trunk here assume a ramified and convoluted form, so as to constitute reservoirs capable of holding a large quantity of pure blood; while the venous trunks exhibit similar dilatations, capable of receiving and retaining for a considerable time the impure blood which has circulated through the system, 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 example 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 latter 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., b 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 ventricle. 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 aerated 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. FIG. 6.-CIRCULATION IN h, heart, inclosed in peri- circulation. 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, etc.) 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. 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 vena 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 dorsal vessel, but analogous to the aorta of mammals and birds, in so much as it supplies the whole body with arterialized blood. After passing through the systematic capillaries, the blood returns in a venous condition to the heart, and the above process 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 h, heart, inclosed in peri cardium; a, the auricle; v, the ventricle; c, the capillary circulation in the gills; d, the dorsal artery; e, the systemic capillaries; b, the veins. in various directions. here conveyed to all parts of the system; We can only allude very briefly to the In the mollusca, we find hearts of 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 consists of trunks emerging 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 branchio-cardiac 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 show, 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 biood to particular parts-is most *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. 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 in. 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, Prof. 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 cold-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 fetus 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. Prof. Draper and others have endeavored to explain it on the principles 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 in. 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 in. 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 there is much muscular movement. The movement of blood through the veins is undoubtedly mainly due to the vis a 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 Prof. Wharton Jones "on the rhythmical contractility of the veins of the bat'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, CAPILLARIES, 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 latterly 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 (later). Many physiologists Circus. 12 dislike the term circulation applied to sap, as suggesting a closer analogy than really exists to the circulation of the blood in animals. CIRCUMCELLIO'NES, fanatical Donatists of the 4th c., who got their name from See PLANT, LEAVES, and SAP. their habits of wandering. They rambled over the country, plundering, burning houses, and murdering those who made resistance, saying that by such means they sought the crown of martyrdom. They styled themselves "Milites Christi Agnostici," and called their chiefs the leaders of the sons of the Holy One. Constantine treated them with forbearance, but under his successor they were put under restriction by the civil power. 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 Abraham, 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 presupposing a knowledge of the rite on the part of Abraham. That it existed 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 practiced. 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 practiced by several tribes in South America. many scholars hold it impossible to suppose that the origin of so universal a rite can Such being the case, 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. edge of C. from the Egyptians, we cannot determine. It would appear, however, that Whether, as Jahn supposes, Abraham obtained his knowlthe 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 seal of the everlasting covenant between God and Israel. 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 Kaflirs, 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 practiced. 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 Egyptiacus, and Winer's Biblisches Realwörterbuch. CIRCUM FERENCE, or PERIPH'ERY, the curve which incloses 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. CIRCUMLOCUTION OFFICE is the name applied by Dickens in his novel, Little Dorrit, to the delays in transacting public business. departments in the art of perceiving how not to do it." He describes it as the "chief of public CIRCUMNAVIGATION, the term usually applied to the act of sailing round the world, its literal meaning being simply a sailing round. considered a great feat, but it is now regarded as one of the most commonplace affairs The C. of the globe was at one time 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 capt. James Cook, who, between 1768 and 1779, made three voyages round the world. CIRCUMNUTATION is the name of a law formulated by Darwin, resulting from observations of Gray, Palm, Dutrochet, von Mohl, and others with his own. waxing parts of plants are governed by a law causing them to rotate as they grow, The and identical with the spiral movement of climbing plants. Experiments show that the movements of the growing parts of all plants are governed by C., while other movements are referred to natural selection. The discovery of C. has scientific significance. CIRCUMPO'LAR STARS, those stars which, in the apparent daily revolution of the sky, do not pass below the horizon of the observer, or, in familiar language, do not set. It will be remembered that the apparent daily motion of the stars is the reflex of the actual rotation of the earth upon an axis which passes through the center of the earth and a point in the sky, near the north, or polar star; that the lines in which the stars seem to move, called lines of daily motion, are the circumferences of circles that are perpendicular to this axis. Hence, if an observer is at the equator, the axis lies in the observer's horizon, the circles of daily motion are all perpendicular to the norizon, and all stars seem to rise and set. If the observer is at a distance from the equator, for example 10° n., the northern end of the celestial axis is raised 10° above the horizon, and any star which is within 10 of the n. pole of the sky will not pass below the horizon in its apparent motion about the pole. The largest circle of the sky which may be drawn about the pole without passing below the horizon, is called the circle of perpetual apparition. A similar circle drawn about the s. pole, without coming above the horizon, is called the circle of perpetual occultation, and the stars within that circle are never visible to the observer in consideration. But, to an observer in the southern hemisphere, having a s. latitude equal to the n. latitude of the first supposed observer, the terms will be transposed; the circle about the s. pole is to him a circle of perpetual apparition, and the stars within it, circumpolar stars. See ASTRONOMY. CIRCUMSTANTIAL EVIDENCE. See EVIDENCE. CIRCUMVALLATION, 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 consists 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 that at Sebastopol, where, while a circuit of batteries fired upon the town, an outer circuit of redoubts and lines kept off the Russians who were in the open field; but the necessity for this arose out of the smallness of the besieging force compared with that of the besieged. The narrow escape of the allies from utter overthrow at Inkermann, showed the necessity for this external defense. For the relation which C. bears to COUNTERVALLATION, see that article. CIRCUMVEN'TION. See FRAUD. CIRCUS. See HARRIER. A CIRCUS, THE, of ancient Rome, was a large oblong building adapted for chariotraces and horse-races, and used also for the exhibition of athletic exercises, mock-contests, and conflicts of wild beasts. The circensian games were alleged by tradition to have originated in the time of Romulus, when they were dedicated to Consus or Neptune, and called Consualia. After the first war undertaken by Tarquinius Priscus, in which he captured the Latin city of Apiolæ, his victory was celebrated by games. space was marked out for a C., and the senators and knights were allowed to erect scaffoldings round it for themselves. The games continued to be held annually, and a permanent edifice was soon afterwards constructed. This was distinguished, subsequent to the erection of the Flaminian and other large circi, as the circus maximus. It must have been altered and enlarged at various times. According to different computations, it was capable of holding 150,000, 260,000, or 385,000 persons. Its extent also has been variously estimated. In the time of Julius Cæsar, it was three stadia, or 1875 ft. long, and one stadium, or 625 ft. wide, while the depth of the buildings surrounding the open space was half a stadium, or about 312 feet. All the circi in Rome, of which there were a considerable number, are now completely destroyed; but a small C. on the Appian way, about 2 m. from Rome, known as the circus of Caracalla, is still in a state of pre servation. Its construction is believed to have differed very little from that of similar buildings. Along the sides and at the curved end were ascending ranges of stone seats for the spectators. At the other end, were the carceres, or stalls, which were covered, and furnished with gates, and in which the horses and chariots remained until, on a given signal, the gates were simultaneously flung open. In the center is the spina, a long and broad wall round which the charioteers drove, terminating at both ends at the meta, or goals-three cones of carved wood which marked the turnings of the course. At each extremity of the carceres is a stone tower. From its gates and castellated appearance, the whole of this side received the name of oppidum, a town. Over the carceres were seats for the president of the games, the consuls, or other distinguished persons. There were four entrances, of which the most important were the porta pompa, and the porta triumphalis. The games were inaugurated by a procession from the capitol, in which those bearing the images of the gods went first, and were followed by the performers in the games, the consuls, and others. This procession entered through the porta pompæ, while the porta triumphalis was that by which the victors left the circus. The spina, an object conspicuous from its situation, was in general highly decorated by such objects as statues, small temples, altars, etc. In the spina of the Circus Maximus, two very large obelisks were erected by Augustus and Constantius. This C. was |