ClKd LATION 



261 



vena cava, goes through the liver, F, in what in 



knov.il us i in- hepatic-portal system. 

 Before passing to tiie comparative study of the 



circulation, we mu*t notice a f-\v of the above 

 fact* in greater detail. 

 The Heart (q.v.) in 

 described in a separate 

 article. Since all the 

 !';' arterial blood leaves 

 tin- heart through the 

 aorta, to trace the 

 circulation of the imre 

 blood involves follow- 

 ing the branches of 

 that great vessel. For 

 this see AORTA. It 

 is sufficient, without 

 further anatomical de- 

 tails, to say that the 

 ramifications of the 

 arteries distribute the 

 arterial blood to the 



Capillaries ( q.v. ) 



Fig. 2.- Theoretical Section 



of the Human Heart : 



/, 6, the two vence caw, opening which pervade every 

 into rf, the right auricle ; c, the par t of the body. It 

 tricuspid valve; a the right [ a l rea< iy evident 

 ventricle, from which proceeds *, ^u^^ i 



the pulmonary artery, dividing tliat the pulmonary 

 into branches g and i, going to arteries from the right 

 the right and left lung respec- ven tricle carry impure 

 tivelv '. e. f. tin- im iiionarv veins . t ,. ii_* i * 

 (two from either lung) enter- blood to the lungs, 

 ing into the left auricle, Jfc ; I, and that pure blood 

 the mitral valve ; 'm, the left returns from the lungs 

 ventricle, from which proceeds t th j ffc aur i c l e b y 

 the aorta, whose arch is nidi- l , "V 



cated by h, and the descend- the pulmonary veins, 

 ing portion by n, none of its The veins, like the 

 branches being indicated in this arte ries, are found in 

 figure ; o, the partition, or sen- , 



turn, between tile right knd left "early every tissue ; 

 hearts. they commence in 



minute networks 



which communicate with the capillaries. Branches 

 from these networks uniting together, form veins, 

 which, by joining, increase in size as they pass 

 onward towards the heart. If we except certain 

 venous structures (called sinuses) occurring in the 

 interior of the skull, we may divide the veins into 

 two sets the superficial or cutaneous, and the deep 

 veins. The deep veins accompany the arteries, and 

 are usually inclosed in the same sheath of tissue. 

 In the case of the smaller arteries they generally 

 exist in pairs, one on each side of the artery, 

 while the larger arteries have usually only one 

 accompanying vein. The superficial veins occur 

 immediately beneath the integument ; they not 

 only return the blood from the skin and adjacent 

 structures, but communicate with the deep veins. 

 All the veins finally lead by two large trunks, the 

 superior and inferior vena cava, into the right 

 auricle of the heart. The superior vena cava is 

 formed by the union of the veins from the head 

 and neck (the jugulars) with those from the arms 

 { the subclavians ), while the inferior vena cava 

 brings back the blood from the lower extremities, 

 the trunk, and the viscera. 



We must refer to the article VEIN for the struc- 

 ture of the walls of this part of the circulatory 

 system. There is only one point that imperatively 

 requires notice here viz. that while the arterial 

 system presents no valves except at the points 

 where the two great trunks leave the heart, the 

 veins contain a great number of valves, which are 

 formed by a doubling of their lining membrane, 

 and resemble pocket-Tike folds or pouches, which 

 allow the blood free passage toward the heart, but 

 prevent its reflux. The veins are much less elastic 

 than the arteries, and their total capacity is much 

 greater. 



There is one part of the venous circulation which, 

 from its great importance, requires special notice 



viz. that of the spleen, pancreas, stomach, and 

 intestinal canal. The blood supplied to these 

 organs by the cteliac and the two mesenteric 

 arteries is not returned directly to the inferior 

 vena cava, but passes by several veins into one 

 large vessel the portal vein, which enters the 

 liver, and breaks up into a capillary network. 

 There the blood undergoes important changes 

 associated with the bile-secreting and glycogen- 

 forining functions of the liver. The blood, enter- 

 ing the liver from two sources, from the portal 

 vein and from the hepatic artery, leaves it by the 

 hepatic veins, which join the inferior vena cava. 

 It is also important to notice the entirely distinct 

 set of vessels known as lymphatics, which conduct 

 the products of digestion into the veins (see 

 LYMPH ). 



The above-described double circulation (through 

 the lungs and through the body) is exhibited by 

 the blood from the time of birth during the whole 

 period of life. The circulation of the Wood, how- 

 ever, begins before birth indeed, at a very early 

 period of intra-uterine or foetal existence ; and 

 the circumstance that before birth the lungs do 

 not act as organs of respiration induces a very 

 important modification in the course of the blood 

 in foetal life which will be described under FCETUS. 



II. Comparative. The circulator}' system in man, 

 as above described, may serve as type of the highest 

 development, differing but slightly from that of 

 other mammals, or that of birds. It is convenient 

 now to begin at the other end, and to note briefly 

 the salient steps of progress in the gradual evolu- 

 tion of the system throughout the animal series. 

 In the unicellular animals the movement of the 

 protoplasm and the special activity of ' contractile 

 vacuoles,' represent, to some extent at least, a 

 circulatory function before the appearance of any 

 system. The canals which so completely irrigate 

 a sponge, likewise illustrate in low expression a cir- 

 culatory system not yet separated off from the others. 

 In Coslenterates, too, the system is still unseparated : 

 4 gastro- vascular' prolongations of the alimentary 

 cavity penetrate the body, as may be very well 

 seen in the disc of a common jelly-fish. In the 

 lower worm-types also, where no distinct body- 

 cavity is yet developed, the nutritive fluid simply 

 diffuses through the body, and no vascular system 

 is differentiated. But in higher worms there is 

 generally a body-cavity, and with it the gradual 

 appearance of a definite vascular system. In some 

 we simply find a fluid moving in the body -cavity, 

 occasionally clear, usually with corpuscles ; in 

 others, portions of the body-cavity are separated 

 oft' as blood-spaces, or eventually as blood-vessels ; 

 these may remain in connection* with the general 

 cavity, or may at length form a closed system. 

 The manifold worm-types afford abundant illustra- 

 tion of all the stages in this differentiation. In 

 the bristle-footed worms (Cha-topods, q.v.), and 

 in some others, the perfecting of the blood-driving 

 mechanism may lie instructively traced. Often a 

 dorsal vessel is'diffusely contractile, less frequently 

 the ventral ; or there may l>e contractile connecting 

 loops between dorsal and ventral vessels, as in the 

 earthworm ; or lastly, a special region in the 

 dorsal vessel may become the main seat of the 

 vascular contractility. Such a tlursal heart is 

 found from this point onwards throughout the 

 Arthropods and Molluscs. (The well-developed 

 and very difficult vascular system of Echinoderms, 

 which co-exists with an abundant body-cavity 

 fluid with relatively few corpuscles, has no special 

 interest for this general survey. ) In crustaceans, the 

 dorsal heart, usually inclosed in a special space or 

 1 pericardial sinus/'drivea blood by more or less 

 well-developed arteries through the body. The 

 rest of the system is best described as lacunar. 



