Reptiles

The ovaries are always paired, and large owing to the size of the yolky eggs. Interstitial tissue is small in amount. The oviducts have independent funnel-shaped ostia, and are usually provided with glandular walls which secrete the albumen and shell. In some viviparous forms they can combine with the faetal membranes to form a placenta. The elongated testes are connected with an epididymis of mesonephric origin.

Respiratory System.

All reptiles breathe by lungs. These are always produced by the elaboration of a median ventral out growth of the pharynx. The glottis lies immediately behind the tongue and is sometimes protected by a rudimentary upstanding epiglottis. There is a larynx, supported by arytenoid and cri coid cartilages, there being no thyroid cartilages; muscles passing from the laryngeal cartilages to the "hyoid" enable the glottis to be opened and closed. There are often vocal chords, which give to Sphenodon, crocodiles, some tortoises and lizards a voice, usually a grunt or squeak. The trachea is often long and its cavity is kept open by cartilaginous rings. The bronchi may be very short in Sphenodon or very long in tortoises. The lung is very variable in its structure; it may be almost as primitive as in Amphibia or become comparable to that of a bird.

In Sphenodon and snakes the cavity of the lung is single, but the walls are divided up into a series of cells by upstanding ridges or septa. In some lizards certain of these septa elongate so that the original single sac begins to be cut up into lobes, each with cellular walls. In crocodiles, this process has gone on so far that the lung is definitely divided into a number of cham bers each of which receives a number of wide side canals, the parabronchi, in whose walls lie the alveoli. In Chelonia this process has gone so much further that the whole lung is spongy, the alveoli, through whose walls the whole of the respiratory exchange takes place, being connected with an irregularly branched series of bronchial tubes.

Not only is the actual structure of the lung altered in this way, but reptiles show an advance over the Amphibia in an increased size of the lung resulting from the development of a special anterior projection, the prebronchial part, which, very small in Sphenodon, becomes much more extensive in more advanced rep tiles. In chameleons, long, hollow non-respiratory process of the lungs pass backward among the viscera; they are important as morphological forerunners of the air sacs of birds and Ptero dactyls. In elongated legless reptiles one of the lungs is usually reduced, and may be absent.

The mode of respiration in Reptilia is not well known. In all except the Chelonia, movements of the ribs may be expected to draw air into the lungs, whilst the muscular post-hepatic dia phragm of crocodiles is, no doubt, used as is the comparable structure in mammals. In Chelonia, and probably also in other reptiles, air is actively forced down into the lungs by movements of the floor of the buccal cavity brought about by the hyoid and its musculature. In Chelonia, the protrusion and withdrawal from the shell of the neck and legs gives a pumping action which, by creating a virtual vacuum, draws air into the lungs.

Vascular System.

The heart of reptiles lies in the thoracic region, usually between the lungs. There is a sinus venosus, at least in most, which opens by a valve guarded slit into the right auricle. Right and left auricles are completely separated, and open independently into the ventricle or ventricles. The lower edge of the interauricular septum is expanded laterally into, usually, very large right and left membranous valves, which direct the arterial blood to the left, the venous to the right side of the ventricular cavity. The ventricle is incompletely, or, in Crocodilia, completely divided by an upstanding ridge into right and left halves. Except for a possible relic in Sphenodon, there is no trace of a conus arteriosus nor of a truncus. Three arteries arise independently from the ventricle, and are then twisted round one another like a rope, so that they cross one another. That vessel whose origin is most to the right is the left systemic arch, the next is the pulmonary arch, and the third is the right systemico carotid, from which arise both carotids. As a result of this ar rangement, in Chelonia the pulmonary arch arises from a partially separated cavum pulmonale, and the two systemics from a cavum venosum, which has to be traversed during systole by the blood from the left auricle, which is originally discharged into the cavum arteriosum on the left side of the ventricle. In crocodiles, the right systemico-carotid alone leaves the left ventricle, whilst both pulmonary arch and left systemic come off from the venous right ventricle : but in them the left and right systemics are con nected by a special opening, the foramen of Panizzi at the point where they cross. Although it has been shown by the electrocardi ograph that the nature of the contraction of the heart in tortoises is much as in mammals, very little is known of its general physiology.