Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/477

Rh VASCULAR SYSTEM,] REPTILES 459 -, ,- The Crocodilia have no epiglottis, but the larynx is attached by u tendinous fibres to the front part of the shield-like basihyal. The larynx consists of a ring-shaped principal cartilage and arytenoid cartilages. There are folds which serve for vocal cords. The trachea is long, with two short bronchi. The former is generally straight, but may be contorted. The more anterior tracheal cartilages are incomplete dorsally ; those of the hinder part and of the bronchi are generally complete. Each lung consists of pouches or cells, the cavities of which open into each other, and com- municate with the cavity of the bronchus by the lateral openings just mentioned. The lungs are enclosed in pouches of peritoneum, and lie, freely suspended, in the body cavity. The Vascular System. r The vascular system of Reptiles consists of a sanguine- ous and a lymphatic system, and the former is further subdivided into an arterial and a venous system, as in the higher Vertebrates, though the latter two are less completely differentiated. In all there is a heart, which consists of at least three distinct cavities, namely, two auricles (which are almost always distinct) and a ventricle, which latter may or may not be completely subdivided into two dis- tinct chambers, a right and a left ventricle. Reptiles differ from the higher Vertebrates in the heart always giving rise to at least two great vessels, which ultimately join to form a single vessel, and from these all the arteries are given off. Most of the blood of the hind limbs and tail passes either through a portal system in the liver, or else in the kidney, on its road to the heart. That which goes to the liver goes to it for the most part through the anterior abdominal veins or vein. The venous blood is conveyed to the right ventricle through a rhythmically contractile sinus venosus. The lymphatic system is well developed, with large reservoirs, and communicates freely with the venous system by a pair of rhythmically contractile so- called "lymphatic hearts." The Heart and Arterial Si/stem. In all the existing Reptiles except the Crocodilia that is to say, in. the Rhynchoccphalia, Laci'.rtilia, Ophidia, and Chelonia the two auricles are divided by a complete septum (except in some Chelonians), but the ventricle is only imperfectly divided into aright and left chamber. Of these the latter is the narrower, the more dorsal in position, and mostly lias thicker walls, and communicates directly with the left auricle. ie right chamber of the ventricle is broader, more ventral, and >sterior, and mostly with thinner walls, and communicates irectly with the right auricle. No arteries proceed from the left chamber, but three proceed from the right chamber, one of which, the pulmonary artery, goes to the lungs, and the other two con- stitute the trunk roots of the whole systemic set of arteries or the two aortae. Between the mouths of their trunk root and that of the pulmonary artery a muscular ridge or valve extends, which Imperfectly divides the right chamber of the ventricle into two cavities, which, during the latter part of the ventricle's contrac- tion, become completely divided by it so as to shut off the con- tained blood from any further access to the pulmonary artery. These great arterial vessels thus leaving the heart are at first closely connected together and are invested with the pericardium, and sometimes, in Chelonians, their investment has been observed to contain striated muscular fibres, thus reminding us of the " bulbus arteriosus " of Amphibians. Afterwards the great arteries proceed in diverging and opposite directions, one aorta arching over to the right and the other to the left, and then meeting and uniting on the dorsal side of the heart to form by their junction the single great dorsal aorta. Amongst the Laccrtilia, the Monitors are remarkable for the almost complete septum between their ventricular chambers. In many Lizards the great arteries going to the head may be so con- nected with the two primitive arterial roots as to form two arches called aortic arches on either side, but in the Monitors, Chameleons, and Amphisbssna these complications are wanting, there being but a single aortic arch on cither side. Generally the right trunk root, or right fundamental aortic arch, after supplying small arteries to the heart itself, gives off a common carotid artery, which divides into a right and a left carotid, and these subdivide to supply the head and neck. It also gives off the snbclavian arteries for the fore limbs- and arteries to the trunk. The left trunk root, or left fundamental aortic arch, gives off arteries to the viscera, and then anastomoses with the right trunk root, the two anastomosing to form the single dorsal aorta, which passes backwards beneath the bodies of the vertebra?, and gives off branches to the'stomach and the viscera, and to the kidneys, sexual glands, hind limbs, and tail. a and es. In the Ophidia the heart is elongated and very distant from the head, and there are of course no linib arteries given off from the aortae. The heart is also distant from the head in the Chelonia, but it is broad in shape and lies above the coracoids. Striated muscular fibres invest the aortic roots in Emys. In the Crocodilia the ventricle is subdivided by a complete septum into two thoroughly distinct chambers, and a trunk root or fundamental aorta proceeds forth from each, the left aorta and the pulmonary artery proceeding from the right chamber, and the right aorta from the left ventricle. Nevertheless, though there is here no communication between the chambers whence these two aortre arise, there is a communication between these two aortae themselves, and that not only when they anastomose after arching different ways, as in all other Reptiles, but also by a direct com- munication between them. When they cross one another, as the aorta from the right chamber arches to the left and that from the left chamber arches to the right, a small aperture places their cavities in communication just outside the heart and close to their respective origins. The Venous System. The systemic veins arising in all parts of Veins, the body collect together and anastomose till their three finally formed trunks open into a more or less capacious contractile cavity, the sinus venosus, which communicates with the right auricle. The anterior two of these three final trunks are called venae anonynise, and the single posterior one is called the vena cava. The former are formed by the gradual junction of the veins of the head, neck, fore limbs, and vertebral region (jugular veins, sub- clavian azygons veins, &c.), and the vena cava is formed by the junction of the iliac veins or those of the hind limbs, together with the veins of the rest of the body. The veins of the alimentary canal, spleen, and pancreas, and the abdominal veins (running between the peritoneum and the abdominal muscles) or vein there being one in the Ophidia commonly collect and unite to enter the liver to form a portal system. In the Ophidia the veins of the alimentary canal, generative organs, fatty appendages, pancreas, and spleen thus unite, while the caudal vein with some veins of the generative organs and intestine go to form a subordinate secondary circulation in the kidney or reni-portal system. In the Lacertilia the veins of the hind limbs collect in part to form a reni-portal system, and also communicate with the abdominal veins, which join the ordinary visceral veins to form the true portal system. In the Chelonia the veins of the tail and hind limbs join the abdominal veins above the plastron, and there with others from the bladder and viscera go to form the portal system, while small branches from the abdominal veins join others from the generative organs and vertebral veins to form a reni-portal system. In the Crocodilia the caudal vein divides on entering the trunk and joins the posterior limb and body veins, and goes on to form, with the visceral veins, a portal system, giving off in this way veins to the kidneys to form the reni-portal system. In all Reptiles the veins of the lungs collect together and empty themselves into the left auricle as they do in all other air-breathing Vertebrates. The Lymphatic System and Vascular Glands. In Reptilia Lym- generally there is a pair of lymphatic hearts placed over the phatic transverse processes of the vertebra at the junction of the tail Avitli system, the trunk. Each usually opens with a small vein which communi- cates with the iliac vein. Besides these bodies, more or less considerable lymphatic canals follow the course of the great arterial trunks in the body and tail, and, when there are limbs, communi- cate with the iliac veins. Other canals accompany the aortic arches and approach and open into the venre anonymse, and others extend backwards towards these from the head and anterior limbs. A pair of bodies which answer to the thymus gland of Mammals lie close to the jugular veins and lymphatics at the anterior part of the thoracic cavity. They arc elongated in Ophidians and roundish in other Reptiles, and they are much larger in young than in old individuals. A thyroid exists in front of the pericardium on the ventral side of the great arterial trunks. It is bilobed in the Crocodiles. The supra-renal capsules are yellowish bodies which lie more in connexion with the generative glands than the kidneys. They are very elongated in the Ophidia, flattened in the Chelonia, and roundish in the Crocodilia. The Nervous System and Organs of Sense. Reptiles, in common with other Vertebrates, have a Nervous nervous system divisible into an axial portion or neural system. axis made up of brain and spinal marrow, and a peri- pheral portion made up of the multitude of nerves which proceed from or are connected with this, and one portion of which is more or less distinctly separable and known as