262 



CIRCULATION 



The venous blood passes along body-cavity spaces 

 to the gills for purification, thence returns to the 

 pericardial sinus, and entering the heart is redis- 

 tributed. In insects a chambered dorsal heart, 

 inclosed in a sinus as before, drives the blood 

 forward, but as one would expect from the very 

 efficient respiratory apparatus, the general vas- 

 cular system is but slightly differentiated. The 

 blood, purified by diffusion from the everywhere 

 present air-tubes, passes back by venous channels 

 into the sides of the pericardium and heart. In 

 myriapods, scorpions, and king-crab, the system 

 is more definitely developed, but illustrates 

 no new advance except that of more complete 

 establishment and wider extension of vessels. In 

 molluscs, however, some progress is observable. 

 Except in the Elephants' Tooth Shell (Dentalium), 

 a heart is present, and the arterial system is often 

 very well developed, even to the extent of capil- 

 laries in some cuttle-fishes and snails. Usually, 

 however, the venous blood travels along lacunae, 

 though gradual transitions occur between these 

 and true veins. The blood purified in the gills or 

 pulmonary chamber passes back into a special 

 portion or the body-cavity the pericardium, and 

 thence into the heart. In certain worm-types, 

 several contractile lateral vessels may often be 

 observed to enter the dorsal vessel ; in the Pearly 

 Nautilus, which has four gil Is, four efferent vessels 

 dilating into four indistinct auricles, enter the 

 median dorsal heart or ventricle ; in almost all 

 bivalves the entrant dilatations or auricles are 

 reduced to two, one on each side ; while in most 

 Gasteropods and Pteropods the specialisation has 

 gone further, and the heart consists of a single 

 auricle and a thicker muscular ventricle. The 

 latter drives the blood through the body by a single 

 or double aorta. 



Passing now with equal brevity through the 

 vertebrate series, we notice first that the heart 

 arises as a dilatation no longer of A dorsal, but of a 

 ventral vessel. Up to and including amphibians, 

 the heart begins as a specialisation of the 'sub- 

 intestinal vein ' in the throat region ; in most, if not 

 all higher vertebrates, it arises from the fusion of 

 two vessels. It always lies in a pericardial sac. 



( 1 ) Among the degenerate Tunicata there is con- 

 siderable variety in the vascular system. In one 

 case no heart is present ; in several there are no 

 definite vessels or blood-corpuscles. The main point, 

 however, is that in most cases a tubular ventral 

 heart drives blood to the respiratory pharynx. In 

 all cases where the heart has been observed, the 

 direction of its beats has been seen to undergo 

 reversal at regular short intervals, a phenomenon 

 which has also been noticed as a rarity in certain 

 worm-types. 



(2) The vascular system of the lancelet or 

 amphioxus is of a peculiarly diffuse and undiffer- 

 entiated nature. It has in one sense no heart, 

 in another sense many ; for while there is no main 

 centre of contractility, there are small pulsating 

 dilatations at the bases of the vessels passing 

 to the gill-slits, while the portal vein and ventral 

 vessel in the anterior pharyngeal region are both said 

 to be contractile. In general course, the circulation 

 is like that of a fish ; the blood passes from ventral 

 vessel to respiratory region, thence to dorsal aorta, 

 thence to body, thence by united sub-intestinal 

 veins to the liver caecum, and thence to the ventral 

 vessel from which it started. 



(3) In the Round Mouths ( Cyclostomata ) the 

 typical fish-circulation is established. The mus- 

 cular ventricle drives the blood by a ventral 

 vessel ('ventral aorta') to the gill-sacs; thence 

 the purified blood is gathered into efferent dorsal 

 vessels, which form in uniting the 'dorsal aorta.' 

 The latter gives off branches to the greater part 



of the body, the head -region being directly 

 supplied from the anterior efferent branchials. 

 The blood returns from the anterior and posterior 

 regions into a uniting vessel behind the heart 

 ( ' the sinus venosus ' ), thence into the receiving 

 auricle, and from that to the muscular ventricle. 



(4) It is enough after the above to notice in 

 regard to the fishes proper, that with the excep- 

 tion of the double-breathing mud-fish (Dipnoi), 

 the heart never contains anything but impure 

 blood, that it drives this wholly to the respira- 

 tory organs, and is in no direct degree 'systemic.' 

 The ' dorsal aorta ' supplying most of the body is 

 formed from the union of efferent branchials, 

 and does not arise, as in higher vertebrates, from 

 the heart. It is important to notice that the 

 five or so arches which spring from the ventral 

 aorta are almost all quite alike, and arise 

 (except in Dipnoi) at slight intervals from one 

 another. A great part of the differentiation 

 in higher vertebrates obviously concerns these 

 aortic arches, which are seen in Cyclostomata 

 and fishes in primitive uniformity, but become 

 modified in higher vertebrates into the caro- 

 tids, sub-clavians, aortic arches, and pulmonary 

 arteries. The heart of a fish consists of the sinus 

 venosus or general junction, running transversely 

 behind the heart (persisting hence onwards, 

 except in adult birds and mammals), of the 

 auricle and ventricle, and except in Teleostei 

 of a specialised contractile portion of the latter 



cet <pe 



Fig. 4. Arterial System 



of Amphibian : 

 ra, right auricle; la, left auricle ; 

 v, ventricle ; ca, conus arter- 

 iosus ; c, carotid arteries ; 

 CM, aortic arches ; ao, dorsal 

 aorta ; pa, pulmonary artery ; 

 pv, pulmonary vein ; ra re- 

 ceives venous blood from 

 body ; both the pulmonary 

 arteries enter la. (After 

 Nuhn.) 



Fig. 3. Arterial System 



of Fish : 



H, heart; c and c', anterior and 

 posterior cardinal veins ; a, 

 branchial arteries; R, capil- 

 laries of the branchial ves- 

 sels ; b, branchial veins ; ce, 

 head circle ; ca, carotids ; 

 RA, root of the aorta ; A, 

 dorsal aorta ; E, artery to 

 viscera (coeliaco-mesenteric ) ; 

 N, renal arteries. (After 

 Wiedersheim.) 



known as the conus arteriosus. A dilatation 

 of the beginning of the ventral aorta is dis- 

 tinguished as the bulbus arteriosus. In regard to 

 the general system, it is worth noticing that in 

 fishes (as in amphibians and all reptiles except 

 Chelonians ) there is a renal portal, as well as an 

 hepatic portal system. Veins from the caudal 

 region of the fish come into the same relations with 

 the kidneys as the portal veins do in regard to the 

 liver. The hepatic veins returning from the liver 

 do not unite with the other posterior veins, but 

 enter the sinus venosus independently. Thus fishes 

 have no inferior vena cava. (4a) The Dipnoi are 

 interesting as leading on to amphibians. The 



