to that of the Mammalia, whilst the abdominal diaphragm undoubtedly causes abdominal respiration. We have seen that these crocodilian conditions do not stand quite alone, but are connected with simpler features in the other reptiles. Two recent, very lengthy papers have been written on this subject by I. Bromann (1904) and by F. Hochstetter (1906), besides two in 1902 by G. Butler.
The Heart.
The Heart of all reptiles is removed from the head and is placed well in the thorax, in the Varanidae even a little beyond it. Only in snakes the heart lies headwards from the hilus of the lungs, not caudalwards, generally at about the end of the first fifth of the body. The batrachian conus arteriosus is reduced, one set of semilunar valves guarding the entrances into the truncus arteriosus which now issues directly from the heart. A sinus venosus exists still in Sphenodon and Chelonians, in which it may even receive separate hepatic veins, but in crocodiles, lizards and snakes the sinus as such exists no longer, forming part, of the right atrium. All the hepatic veins enter the stem of the posterior vena cava, which henceforth enters the heart as inferior vena cava. This, the largest, and the right and left anterior vena cavae, are the only three veins which enter the right atrium. Into the left open the two pulmonary veins. Right and left atrium have in all reptiles a complete septum between them. The ventricular portion shows considerable steps towards the differentiation into a right and a left ventricle, but the partition is very incomplete in tortoises, lizards and snakes, quite complete only in the crocodiles. The most important character of the reptilian heart, absolutely diagnostic of it, is the fact that the systemic vessel which leaves the right ventricle turns to the left to form the left aorta, while the stem which comes from the left ventricular half arches over to the right as the right aorta. It is not at all necessary to conclude that this fact excludes the reptiles from the mammalian ancestry and to hark back to conditions as indifferent as are those of the batrachia. The Foramen Panizzae shows the way to a solution, how ultimately all the arterial blood from the left ventricle may pass, first through the root of the right arch, then through this hole into the left. Whilst the rest of the right arch, and the root of the left, obliterate. The difficulty is not much greater than that of deriving the birds' condition from the reptilian. The Foramen Panizzae, which exists only in the Crocodilia, lies exactly where the right crosses dorsally over the left aorta. The whole is not the last remnant of the originally undivided truncus, as is taught generally, but it is a new foramen, a hole dug by the left arterial blood into the venous right aorta. According to the recent observations made by F. Hochstetter the foramen comes into existence in a very late embryonic stage.
Whilst the batrachian single ventricle possesses only one ostium ventricular or outlet into the truncus, in the reptiles the inter-atrial septum extends considerably downwards into the base of the ventricle, so as to produce a right and a left niche, and correspondingly two ostia instead of one. The atrio-ventricular valves are still membranous, even in crocodiles; attached to them are muscles, trabeculae carneae, from the very trabecular walls of the ventricle; they are especially spongy in tortoises. By means of the arrangement of some of these trabeculae, perhaps still more through the confluence of their basal portions, an imperfect ventricular septum is initiated. Certainly even in tortoises, which represent the lowest stage, the venous blood is received into and sent out by the same right side of the ventricle, while the arterial blood is correspondingly managed and dodged by the left side. That there is not very much mixture of the two kinds of blood, in spite of the wide communication in the ventricle, is further due to the peristaltic systole and diastole of the various divisions of the heart.—The heart of Chelonians is broader than long. In correlation with the very much flattened body of Trionyx and its allied genera, the whole heart is dislodged from the middle line, far over to the right side; the vessels of the left side are correspondingly much elongated and have to cross the neck, trachea and oesophagus.—The apex of the heart is attached to the pericardium by a special ligament in the Crocodilia and in many Chelonia, e.g. Testudo, but it is absent in Clemmys. Sometimes this little ligament sends a tiny blood vessel into the liver.
Arterial System.
Crocodiles.—The left aorta crosses obliquely beneath the right and gives off only the coeliac, just before joining the right aorta in the level of the eighth thoracic vertebra. The aorta descendens sends off, besides intercostals and other segmental into the body-wall, the mesenteric, right and left iliac, a pair of renal and ischiadics, a cloacal and the caudal artery. The right aorta forms the main root of the a. descendens. Close to the heart it sends off two coronaries and a short carotis primaria which divides at once into two anonymae, the left of which is the stronger. The right anonyma divides into the subclavia and collateralis colli, the left into subclavia and carotis subvertebralis. Each subclavia sends off an a. vertebralis communis, which runs head wards and, with another longer branch, downwards, giving off intercostals, and then joins the descending aorta.
Tortoises.—The left aorta is rather more separated from the truncus, which it crosses ventrally in an oblique forward direction; it sends off a left cardiac to stomach and oesophagus, a coeliac and mesenteric, and then a communicating branch to the right aorta. The a. descendens gives off paired suprarenals, spermatics, very large iliacs, then a pair of renals, hypogastrics and the caudal. Each iliac artery divides into a recurrent intercostal anastomosing with the axillaries, an epigastric (sending off the crural and anastomosing with thoracic and humerals), and other arteries to abdominal muscles and to the shell. The hypogastrics supply the cloacal region and then continue as the ischiadics. But there are many anastomoses which cause great variation in the different tortoises. The right aorta sends off a right cardiac, the coronary, and the right and left anonymae which are quite symmetrical, each dividing into subclavia and carotis; in the angle lies the thymus.
Lizards.—Two common carotids arise either side by side, or by one carotis primaria, from the right aortic root. In the majority each common carotis ascends the neck and then divides into the vessels for the head and another branch which turns back and goes into the descending part of the aortic arch. In chameleons two carotid stems ascend the neck and there is no recurrent vessel. In the Varanidae the two common carotids start from a long carotis primaria, there is no recurrent vessel. The vertebral arteries come from the origin of the subclavians and run to the head in a very lateral position. The subclavian arteries (which occur also in limbless lizards) arise far away from the carotids out of the descending arch of the right aorta, in a level often far behind the heart. “Anonymous” arteries are consequently absent in lizards.
Snakes.—The left aorta is stronger than the right, both combining soon to form the descending aorta. Owing to the absence of fore limbs and shoulder-girdle the conditions are much simplified. In most snakes the right aorta sends off but one strong carotic vessel which represents the left carotis communis whilst the right is much reduced or even quite absent; further, there is only one vertebral artery, which either runs along the right side of the vertebral column or it divides soon into a right and a left vessel along the neck. In conformity with the reduction of one lung there is usually but one pulmonary vessel.
Venous System.
Crocodiles.—Each, right and left, anterior vena cava is composed of a subclavian (axillary and external jugular), an internal jugular, common vertebral and an internal mammary vein. The posterior vena cava is composed of the two revehent renals, veins from the genital glands and ducts, revehent veins of the suprarenals (which, like birds, still have a portal system), and the big vein from the fat body. Thus the vena cava posterior
