CIRCULATION 



heart receives pure as well as impure blood, drives 

 l.l..d to IUIIM, in iulilitiiiti to gills, in in part 

 systemic (driving mire blood in rrotopterus and 

 Li-pidoMii'ii through the flint two pairs of arches), 

 has the roots 'i the aortic arches close together, 

 and is |>nicticiilly three-chambered. In these 

 respects it reaches forward to the condition seen 

 in those amphibians \vliich retain their gills. 



(5) The heart of amphibians is three-chambered, 

 a right auricle receiving impure blood from the 

 body, a left auricle receiving purified blood from 

 the lungs or from the lungs and gills, a single 

 ventricle which drives the blood, mingling less 

 than mL'lit ! supposed, to head or body or 

 respiratory organs. In some cases, as one would 

 expect, the partition Itetween right and left auricle 

 is imperfect. In the tadpoles at the fish-like stage 

 there are four aortic arches, of which the first three 

 supply gills. Where gills persist, such supply is 

 always of course in some degree maintained. In 

 such a metamorphosis as that of the frog, where all 

 trace of gills is lost, the first branchial arch becomes 

 the carotid ; the second is the systemic, which 

 forms, by uniting with its fellow, the dorsal aorta ; 

 the third dwindles away ; the fourth supplies the 

 lungs. The same is generally true of tne higher 

 vertebrates, except that it is usually the third 

 branchial which forms the pulmonary artery. An 

 inferior vena cava is definitely established in amphi- 

 bians ; there is a renal portal as well as an hepatic 

 portal system ; special lymph hearts are sometimes 

 present connecting the lymph system with the 

 vascular. 



(6) Among the reptiles, the differentiation of 

 the heart goes a step further. In Chelonians, Lacer- 

 tilia, and Ophidia, a strong muscular ridge forms 

 an incomplete partition, dividing the ventricle into 

 A right portion containing purely venous blood, 



Fig. 5. Circulation of a Reptile (Tortoise): 

 ro, right auricle : la, left auricle; rv, right (venous) portion of 

 ventricle; Iv left (arterial) jwrtion of ventricle; loo, left 

 ( venous ) aortic arch ; pa, pulmonary artery ; moo, right (arterial ) 

 aortic arch ; r, Is, branches to fore-limbs ; cc, carotids ; pv, 

 pulmonary veins; tvc, superior venie cavK ; ivc, inferior vena 

 cava; ao, dorsal aorta ; cu-l, coeliac artery to viscera. (After 

 Nuhn.) 



'and a left portion containing mixed and arterial 

 blood. The pulmonary artery rising from the right 

 cavity takes purely venous blood to the lungs ; of 

 the two aortic arches rising from the left cavity, 

 the left aorta contains more venous than arterial 

 blood, the right aorta more arterial than venous. 

 In Crocodilia, however, there is a complete septum 



in the ventricle, and thus for the tint time a four- 

 chambered heart, though it is not certain that the 

 division is exactly comparable to that of bird* and 

 mammals. From the right or venous ventricle rue 

 the pulmonary artery and left aortic arch ( taking 

 venous, not mixed, blood to the viscera) ; from the 

 left or arterial ventricle rises the right aorta, with 

 pure blood to the greater part of the Txxly. Though 

 the division of arterial and venous chambers brings 

 the crocodilian heart to the bird or mammal level, 

 there are two aortic arches, one venous, the other 

 arterial, which unite as usual, and also communicate 

 by a foramen at their roots. In all reptiles there 

 are two superior vence cava;, an inferior vena cava 

 t'ormed from the union of the two efferent renals, 

 and a renal portal system except in Chelonia. In 

 birds and mammals a single aortic arch forms the 

 dorsal aorta, not two or more as in reptiles. In birds 

 the aorta goes to the right, in mammals to the left, 

 I HI t except in this variation and in the structure of 

 the Heart (q.v.), the circulation in Birds (q.v.) and 

 mammals (supra) is practically identical. 



III. Physiology. The most important conditions 

 of circulatory function are ( 1 ) a general constancy 

 and rapidity of How, and (2) a power of adapting 

 this to special needs. The conditions of the former 

 are mainly mechanical and physical those of the 

 latter depend upon the nervous system. 



( 1 ) The Mechanism of Circulation chiefly depends 

 upon the rhythmic contractility of the heart, the 

 elasticity of the vessels, and the friction in the 

 small arteries and capillaries. The heart is a 

 muscular pump contracting under the influence of 

 its automatic nervous mechanism, but also respond- 

 ing in the nature of its beat to the conditions of 

 the body. The human heart usually beats about 

 seventy-two times a minute, and in the eight- 

 tenths of a second occupied by each beat, three 

 distinct events occur the contraction (systole) of 

 the ventricles, a ' passive interval ' of relaxation 

 ( diastole ) of both auricles and ventricles, and the 

 decidedly briefer contraction of the auricles. The 

 pumping action of the heart is thus obviously an 

 intermittent force which drives the blood through 

 the closed series of elastic tubes formed by the 

 blood-vessels. To the latter attention must now 

 be directed. 



The arteries are surrounded by muscular tissue, 

 and are contractile as well as h'ighly elastic. As 

 they break up into branches from the aorta to the 

 capillaries, the capacity for holding blood is con- 

 tinually increasing. The flow of blood within 

 them, though continuous, comes in gushes corre- 

 sponding to the heart-beats. The velocity of the 

 stream is greatest the nearer the heart. Tne veins 

 are much less elastic than the arteries, and have a 

 much greater total capacity for holding blood. As 

 they unite from the capillaries to the venae cavae, 

 the total capacity is continually diminishing. The 

 flow of blood within them is continuous, but with 

 relatively little force and small velocity. The 

 capillaries have a very small calibre, but are both 

 elastic and contractile. They are permeable by 

 fluids, and also allow the corpuscles to pass through 

 their walls. The velocity of the flow is here at ite 

 minimum. The resistance to the flow of blood, 

 due to the friction of these minute passages, works 

 back along the arterial system to the heart, and is 

 one of the most important factors in determining 

 the nature of the circulation. 



To return now to the actual mechanism, it is 

 only necessary to note (a) that the whole system is 

 always over-Ailed with blood, which consequently 

 causes a pressure on the walls ; ( 6) that the cause of 

 the circulation is the difference of pressure between 

 the blood in the aorta and pulmonary arteries OD 

 the one hand and the venae cavae and pulmonary 

 veins on the other; (e) that this difference of 



