very asymmetrical, one of them being much larger than the other, which is often quite aborted.
The simplest form of lungs is that of Sphenodon; the prebronchial part is still small. Each lung is still a sac with one large lumen, the walls being honeycombed. In the lizards the walls are more spongy, and several septa begin to extend more or less far from the walls into the lumen, towards each bronchus. Some of these septa begin to cut the lung into lobes, especially in Varanus and in chameleons. In the latter exists a further specialization, a side-departure, in the shape of several long, hollow processes which are sent out from the posterior portions of the lungs and extend far into the body-cavity and between the viscera. By means of them these creatures can “blow” themselves out. They are of morphological interest since they are first stages of air-sacs so marvellously developed in birds, and possibly also in various Dinosaurs. In the Amphisbaenids the left lung alone remains.
The lungs of crocodiles have reached a considerably higher stage. They alone in reptiles are, on the ventral side, completely shut off from the viscera by a pleural, partly muscularized, membrane. From each bronchus extend a number of broad septa towards the periphery, dividing the originally single lumen into many chambers, perhaps a dozen, from the walls of which wide secondary or parabronchial canals extend into the alveolar meshwork, in very regular arrangement, in series like organ-pipes.
The lungs of the tortoises are, in adaptation to the peculiar shape of the body, stowed away along the back, as far as the pelvis, and only their ventral surface is covered by a strong peritoneal membrane which receives muscular, diaphragmatic fibres. The inner division of the lungs into chambers has progressed so much that a sort of mesobronchus has become discernible; the arrangement of the side-bronchi is far less regular than in crocodiles; the whole lung is much more honeycombed, meshy and spongy.
The mechanism of breathing of tortoises is not such a puzzle as it is sometimes stated to be. Of course the rigid box of the trunk excludes any costal, or abdominal breathing, but by protruding the limbs or the neck, piston-like, an effective vacuum is produced in the box. Moreover, the throat is distended and worked considerably by the unusually large and very movable hyoid apparatus, by which air is pumped into the lungs.
The lungs of the snakes are very thin-walled, with a very wide lumen, and only for about the first half from the heart backwards the walls are alveolar enough for actual respiratory function, while towards the blind end the sacs are so thin and sparsely vascularized that they act mainly as reservoirs of a large amount of air. Frequently their posterior portions receive blood vessels not from the pulmonary arteries but directly from those of the trunk. In correlation with the long, cylindrical body, the lungs are much elongated and they are not equally developed. The asymmetry shows great differences in the various groups, consequently the asymmetry has been developed independently in those groups. It is usually stated that the left lung is much smaller than the right. This is but rarely the case. The most recent observations are those of E. D. Cope (Proc. Am. Phil. Soc. (1894), xxxiii. 217). In Boidae both lungs are large, although unequal: the left or more dorsally placed one being the larger. In Ilysia the right is functional, the left is ventral and vestigial. In Rhinophis the right is very small, the left larger. In Glauconia and Typhlops the right lung alone is developed: the left is quite aborted. In Colubridae the left lung alone is functional, while the right is vestigial. There is no trace of the right in Elapinae and Hydrophinae and most Viperidae. In the Colubridae the right, or ventral, lung is, when present at all, reduced to a length of from 2-5 mm., and it then communicates with the anterior portion of the left lung by a foramen, in level of the heart, whilst the right bronchus is aborted.
A further complication is the so-called tracheal lung, which is present in Typhlopidae, Ungalia of the Boidae, in Chersydrus of the Acrochordinae, in the Hydrophinae and Viperidae. This peculiar organ is a continuation of the anterior portion of the functional lung, extending far head wards, along the trachea, with the lumen of which it communicates by numerous openings. In Chersydrus this mysterious organ is “composed of coarse cells and without lumen, extends from the heart to the head, and is discontinuous with the true lung; the trachea communicates with it by a series of symmetrical pores on each side.” In Typhlops it extends likewise from the heart to the throat, as a cellular body but without lumen or connexion with either trachea or lung.
Thyroid and Thymus.
The Thyroid of the reptiles is a single, unpaired organ, placed ventrally upon the trachea and one or other of the arterial trunks, more or less distant from the heart. In snakes it lies on the mid-line near the heart; a little farther up in Sphenodon; still farther in lizards, and chameleons near the root of their gular sac. In tortoises it is globular, at the division of the carotic trunk. In crocodiles it is bilobed.
The Thymus is paired. It is largest in crocodiles, extending on either side of nearly the whole neck, along the carotids and jugulars. In the tortoises they are much shorter; in Sphenodon and lizards are two pairs, more or less elongated; in the snakes are sometimes as many as three pairs, elongated but small, attached to the carotis near the heart. As usual the thymus bodies become much reduced with age.
The Spleen.
The Spleen varies much in shape and position. In lizards it is mostly roundish, elongated in Sphenodon, and placed near the stomach; in crocodiles it lies in the duodenal loop behind the pancreas; similarly situated in snakes, but in the tortoises it is much concentrated, large and attached to the hind-gut.
The Body Cavity.
The body cavity of the reptiles is subdivided into several sacs or cavities by serous membranes of peritoneal origin. The number of these subcavities differs much in the various groups. The pericardial sac is always complete. In tortoises the lungs are retro-peritoneal, a dense serous membrane spreading over their ventral surface from the walls of the carapace forwards to the liver and shutting off a saccus hepato-pulmonalis from the rest of the peritoneal cavity. Snakes possess, besides the modifications mentioned above, separate chambers for the stomach, right and left liver, and for the gut, whilst the pleural cavities as such have been destroyed. In lizards a “post-hepatic septum” divides liver, lungs and heart from the rest of the intestines. This transverse vertical septum is best developed, almost complete, in some of the Tejidae, in others it seems to be more imperfect, and it is probably a further development of the suspensorial ligament of the liver, which is ultimately inserted upon the ventral wall of the body.
The subdivisions have reached their highest development in the crocodiles, there being, besides the pericardia and the two pleural cavities and the usual peritoneal room, a right and left hepato-pericardia, an hepato-gastric, and an hepato-pulmonal sac. The caudal and ventral edges of these liver-sacs are fused on to the ventral body-Wall, thus producing a complete transverse partition, head wards of which lie the lungs, liver and heart. This partition, morphologically not homologous with the mammalian diaphragm, more resembling the imperfect structure in birds, acts, however, as a perfect diaphragm, since it is well furnished with muscular fibres. These are attached to its whole periphery, with centripetal direction, especially on the ventral half. These fibres are transgressors upon this septum from a broad sheet of muscles, which, inserted together with the septum upon the body-wall, arise from the iliac bones, the pubes, and the greater portion of the last pair of abdominal ribs. This broad muscular sheet, covering the intestines, is the so-called abdominal diaphragm or peritoneal muscle. Its continuation upon the transverse septum is the crocodilian muse. diaphragmaticus, and in functional effect very similar
