Reptiles

Arterial System.

The pulmonary arch soon divides into two branches, one to each lung; in Sphenodon and some lizards it gives off a paired laryngotracheal artery which is a relic of a into the left auricle.

The venous blood is returned to the heart by the pair of pre caval, and single postcaval veins which open into the sinus ve nosus. The branches of the precavals come from the head and fore limb, the subclavian often receiving an azygous vein from the anterior part of the body wall which represents part of the embry onic posterior cardinal. Nearly the whole of the blood which Urodela structure. The systemic arches unite to form the dorsal aorta; from one or both of them arise coronary arteries to the heart. From the right come off both subclavians, and the left usually gives off a coelic branch. The carotids may arise inde pendently from the right systemico-carotid, or may be formed by the branching of a single primary carotid. In snakes the right carotid is usually much reduced or absent.

Venous System.

The venous system is exceedingly compli cated, differing in details in the four orders but with a common ground plan. The pulmonary veins pass straight from the lungs enters the heart through the postcaval has previously passed through one of the portal systems.

The renal portal system drains the tail, and part of the hind limbs, the afferent renals arising from the bifurcated anterior end of the caudal vein and the iliacs. The efferent renals open into the postcaval, whose hinder end is formed by them. The supra renal portal system consists of a series of afferent veins which come from the body wall; the efferents discharge with the gonadial veins into the postcaval.

The hepatic portal system includes the series of veins from the gut, which form the true hepatic portal vein and also the median anterior abdominal vein, which is originally formed by a fusion in the middle line of pelvic veins, themselves built up from the iliacs and a series of vessels from the hinder part of the body wall of the abdomen. The anterior abdominal passes along in a mesenteric sheet in the ventral part of the body cavity to enter the liver and there receive the hepatic portal or a branch from it. Finally, the whole of the blood in the liver passes by the

hepatic veins into the posterior cardinal.

Lymphatic System.

Definite lymphatic canals are well de veloped in reptiles ; those of the head unite into thoracic ducts which open into the innominate veins. There is a pair of pos terior lymph hearts discharging into the iliacs.

Blood.

The red blood corpuscles are oval, biconvex and nu cleated; they are larger than those of birds and mammals, smaller than those of Amphibia.

"Ductless Glands..

A spleen is constantly present in rep tiles, placed near the stomach or within the loop of the duodenum behind the pancreas. The reptilian thyroid is a median structure placed somewhere on the ventral surface of the trachea.

There are two pairs of thymuses in Sphenodon and lizards, derived from the second and third pharyngeal pouches in the latter. In snakes there are usually two pairs derived from the mammals. The nasal cavity finally opens to the palate by the internal nostril, which may be carried far back by the formation of a secondary palate.

Jacobsen's organ is, in Chelonia, a mere diverticulum of the ventral part of the nasal cavity. In Squamata it becomes an independent chamber, separated from the nasal cavity by the septomaxillary bone; it then has a special opening to the palate and may be very highly developed, receiving a large proportion of the olfactory nerve fibres. Its function is clearly to smell food after it has been taken into the mouth.

Jacobsen's organ soon vanishes in crocodiles. A special nasal gland is developed in the concha of reptiles, and the naso-lachri mal duct opens on the lateral wall of the nasal cavity.

Eye.

The eyes of reptiles are normally provided with movable upper and lower eyelids, and a nictitating membrane which is usually transparent and can be drawn across the cornea. The nictitating membrane may vanish in some lizards ; in snakes it is permanently drawn across the eye, fuses with the remnant of the upper eyelid, and has a cornified scale on its outer surface, which is shed with the rest at ecdysis.