CIRCULATION. 762 CIRCULATORY SYSTEM. to the total amount of blood in the body : the muscular tissue of the arteries is an additional factor here. The arteries also tend to regain their normal calibre under all circumstances, be- cause of this same elasticity, and easily adapt themselves to the different movements of the body. The most important function of the muscular tissue in the arteries is to adjust the size of the vessels to the amount of blood which each part of the body requires, and also to adapt the calibre to the (|uantity of blood that the arteries contain at any moment. The force of the nuiscu- lar contraction in the arteries is probably a very slight factor in propelling the blood. The im- pulse of the blood, when it enters the arteries at the moment of ventricular contraction, can be felt in all the superficial arteries of any con- siderable si?" ; and we designate this as the pulse. F(,r clinical purposes, it is usually felt in the radial artery at the outer side of the wrist. In the mesentery of the frog, we can trace the circulation in the capillaries by means of the microscope; and our observations here can prob- ably, without error, be ap])lied to the condition in man. The red blood-corpuscles are seen mov- ing along with considerable rapidity in the middle of the blond-current, while the white cor- pu.scles advance more sluggishly along the walls of the capillaries. The capillaries present a far larger surface with which the blood comes in con- tact than tw other blood-vessels, and therefore oll'er the greatest resistance to the progress of the blood. This resistance largely depends on the vital capillary force, so that the capillaries of themselves greatly influence the circulation. On the other hand, the condition of the arteries and veins exerts a decided control over the cir- culation in the capillaries which connect them. The propelling force in the veins is due to the impulse from the ventricles, and, to some extent, to the pressure exerted upon the veins by the muscular movements of the body. The suction action of the Jieart is also a factor to be con- sidered. The presence of valves along the veins enables them to take advantage of all the force applied. It is estimated that a portion of the blood can traver.se the entire circuit of the cir- culation in half a minute. The circulation lo- gins at a very early ])i'riod in fietal life, and presents important nioditications, which will be described under Fcetis (q.v.). See also Em- rRYOLocY, Human. Our ])rescnt knowledge of the circulation is of comparatively recent date. It was first de- scribed by Harvey, in his ccdebrated work, Exer- cifatio de Motit Cordis rt Sanguinis, published in 1628. CIRCULATION OF SAP. See Sap. CIR'CULATORY SYSTEM, Evolution of. The organs by wliiili lluicls and sometimes gases and solids, suspended in the fluids, are trans- ported from one jiart of the body to another. A circulatory system first bceomes necessary vhen the organism gets to be of great size and a specialization of structure has taken place, so that the nutrient regions where fluid and gas- eous food arc sup|ilied are far removed from the muscles, glands, and other organs which re- quire that food, and where? the excretory organs are distant from the great centres of metabolism. The circulatory system fulfills the function of transportation between the muscles and glands on the one hand, and the alimentary tract, res- })iratory organs, and kidneys on the other. LonesI Forms. — In the Protozoa the body is so small that no special circulatory apparatus is necessary. The cell -structure with its water- spaces provides for carrying food in solution to the remotest plasm films. In the sponges the body often attains great size, liut by the system of water-canals penetrating all parts of the nuiss. food and oxygen are directly brought, as it were, to the door of every cell. Even in the Cnidaria the body-wall is typically only two cells thick, so that between the external " fluids and those of the cavity every cell is bnniglit into direct contact with oxygenated water. In the l>colecida. where .a true body-cavity between the alimentary tract and the body-wall makes its appearance, the fluids of this cavity serve to carry nutritive lluids and excreted products. The fluids of the body-cavity are often set in motion by the cilia of the lining cells. In the Mollusca the body is so large and tiie nmscular system so well developed that a more effective transporting system is necessary. This is gained by the separation of special tracts of the body-cavity. In the dorsal part of the animal is a space that has gained thick muscular walls and constitutes a propelling organ or lieart. The alimentary tract usually perforates the heart so that nutri- tive fluids by passing through the wall of the gut may reach the circulating fluid. Kroui the lieart a tubular space carries the circulating fluid to the spaces of the mantle and of the muscles and glands. These spaces are parts of the body- cavity and. as in the case of the foot, some of them even connect with the outside world. The fluids in these spaces and in the gills make their way, probably by the aid of cilia, to the heart. Oxygen is obtained both in the thin- walled mantle and in the gills. The Mollusca show thus a great advance in the specialization oT a definite system of blood-vessels. The echino- derms have acquired much the same or a less perfect system of blood-spaces. Arthropods. — Among segmented animals we find that many of the Polyclueta have a well-de- lined system of blood-vessels, which they have perliaps derived from the nemerteans. The system shows an advance over that of nemerteans, how- ever: for in the Latter grouj) there is no regular circulation, whereas in the Polycha'ta such a cir- culation is established. They have a main dorsal vessel and a median ventral vessel, and a circular transverse vessel running in the body-wall be- tween the two. Vessels going to the alimentary tract get nutritive fluids and those going to the body-wall, parajxxlia. and gills bring back oxygen. The blood-vessels contain cor|)Uscles and often h;emoglobin. The dorsal-vessel 'heart' contracts peristaltically. Not all Polych.Tta have a com- plete (closed) blood system, and in some eases the body fluid is the only circulating medium. The earthworms also have a well-develo])ed sys- tem of vessels. In the Crustacea there is usually a dorsal-ves- sel 'heart.' but well-defined arteries and veins are <iften lacking, the heart pumping the general fluids of the bodv-eavity. However, in the higher Crustacea, as, e.g. the lobster, definite vessels to the brain, muscles, gills, and alimentary trai-t appear, from which organs the blood is re- lumed to the heart by blood-spaces. In insects
