CHAPTER X.. Organs of Circulation.

ORGANS OF CIRCULATION.

The Blood-corpuscles of fishes are, with one exception, of an elliptic shape; this exception is Petromyzon, which possesses circular, flat, or slightly biconvex blood-corpuscles. They vary much in size; they are smallest in Teleosteans and Cyclostomes, those of Acerina cernua measuring 1/2461 of an inch in their longitudinal, and 1/3000 in their transverse diameter. As far as it is known at present the Salmonidæ have the largest blood-corpuscles among Teleosteans, those of the salmon measuring 1/1524 by 1/2460 in., approaching those of the Sturgeon. Those of the Chondropterygians are still larger; and finally, Lepidosiren has blood-corpuscles not much smaller than those of Perennibranchiates, viz.--1/570 by 1/941 in. Branchiostoma is the only fish which does not possess red blood-corpuscles.

Fishes, in common with the other Vertebrates, are provided with a complete circulation for the body, with another equally complete for the organs of respiration, and with a particular abdominal circulation, terminating at the liver by means of the vena portæ; but their peculiar character consists in this, that the branchial circulation alone is provided at its base with a muscular apparatus or heart, corresponding to the right half of the heart of Mammalia and Birds.

The Heart is situated between the branchial and abdominal cavities, between the two halves of the scapulary arch, rarely farther behind, as in Symbranchidæ. It is enclosed in a pericardium, generally entirely separated from the abdominal cavity by a diaphragma, which is, in fact, the anterior portion of the peritoneum, strengthened by aponeurotic fibres. However, in some fishes there is a communication between the pericardial and peritoneal sacs, viz. in the Chondropterygians and Acipenser, whilst in the Myxinoids the pericardial sac is merely a continuation of the peritoneum.

a, Atrium; b, Conus arteriosus; d, Papillary valve within the conus; e-g, Transverse rows of Ganoid valves; h, i, Anterior arcus aortæ; k, l, Posterior arcus aortæ; v, Ventricle.]

The heart is, relatively to the size of the body, very small, and consists of three divisions: the atrium, with a large sinus venosus into which the veins enter; the ventricle; and a conical hollow swelling at the beginning of the arterial system, the structure of which forms one of the most important characters used in the classification of fishes. In all Palæichthyes (Figs. 66 and 67) this swelling is still a division of the pulsating heart, being provided with a thick muscular stratum; it is not separated from the ventricle by two valves opposite to each other, but its interior is fitted with a plurality of valves, arranged in transverse series more or less numerous in the various groups of Palæichthyes. Lepidosiren and Protopterus offer an example of a modification of this valvular arrangement, their valves being longitudinal, each valve in fact being formed by the confluence of several smaller ones situated behind one another. This Palæichthyan type is called conus arteriosus.

In Cyclostomes and Teleosteans (Fig. 68) the enlargement is a swelling of the artery, without muscular stratum and without contractility; with the exception of the Myxinoids its walls are thick, fibrous, with many trabeculæ and pouches, but it has no valves in its interior, and is separated from the ventricle by two valves opposite to each other. This Teleostean type is called bulbus aortæ.

a, Section through part of the wall of ventricle; b, Section through the bulbus; c, Teleosteous valves of the ostium arteriosum; d, Accessory valves, of rudimentary nature and inconstant; e, Trabeculæ carneæ of the bulbus.]

The sinus venosus sends the whole of the venous blood by a single orifice of its anterior convexity into the atrium; two thin membranous valvules turned towards the atrium, prevent the blood from re-entering the sinus. A pair of other valves between atrium and ventricle have the same function. The walls of the ventricle are robust, and, internally, it is furnished with powerful fleshy trabeculæ.

The bulbus or conus arteriosus is prolonged into the branchial artery which soon divides, sending off a branch to each branchial arch. On returning from the respiratory organ the branchial veins assume the structure and functions of arteries. Several branches are sent off to different portions of the head and to the heart, but the main trunks unite to form the great artery which carries the blood to the viscera and all the parts of the trunk and tail, and which, therefore, represents the aorta of higher animals.

In the majority of Teleosteans the aorta has proper walls formed by its own membranes, but in the Sturgeons it is independent at its commencement only, and replaced by a canal formed by hæmal elements of the vertebral column, and clothed inside with a perichondrium. In many Chondropterygians and some Teleosteans (Esox, Clupea, Silurus), the aorta possesses its own firm membranes along its ventral side, dorsally being protected by a very thin membrane only, attached to the concavity of the centra of the vertebræ.

The circulatory system of Branchiostoma and of the Dipnoi shows essential differences from that of other fishes.

Branchiostoma is the only fish which does not possess a muscular heart, several cardinal portions of its vascular system being contractile. A great vein extends forwards along the caudal region below the notochord, and exhibits contractility in a forward direction; it is bent anteriorly, passing into another tube-like pulsatile trunk, the branchial heart, which runs along the middle of the base of the pharynx, sending off branches on each side to the branchiæ; each of these branches has a small contractile dilatation (bulbillus) at its base. The two anterior branches pass directly into the aorta, the others are branchial arteries, the blood of which returns by branchial veins emptying into the aorta. The blood of the intestinal veins is collected in a contractile tube, the portal vein, situated below the intestine, and distributed over the rudimentary liver. Of all other fishes, only in Myxinoids the portal vein is contractile. All the blood-corpuscles of Branchiostoma are colourless and without nucleus.

In Dipnoi a rudimentary division of the heart into a right and left partition has been observed; this is limited to the ventricle in Ceratodus, but in Lepidosiren and Protopterus an incomplete septum has been observed in the atrium also. All Dipnoi have a pulmonal vein, which enters the atrium by a separate opening, provided with a valve. The pulmonal artery rises in Lepidosiren and Protopterus from an arch of the aorta, but in Ceratodus it is merely a subordinate branch, rising from the Arteria cœliaca.