REPTILIA. 



307 



marked by Nitzsch, may be considered a rudi- 

 ment of the left lung. The right, and in this 

 case sole, pulmonary sac is placed immediately 

 below the spine ; it extends posteriorly as far as 

 the region of the kidneys, and in the Coluber 

 natrix is from five to seven inches long, and 

 from one-half to three-quarters of an inch 

 broad. Its parietes are thickest at the point 

 where the rings of the trachea cease, where 

 it is covered externally by a fibrous layer, 

 and lined internally by a fine lattice-like net- 

 workof vessels. More posteriorly the pa- 

 rietes become gradually thinner, and at last 

 are merely membranous, giving to the whole 

 organ still more of the appearance of a swim- 

 ming bladder. In the slow-worm (Anguis 

 fragilis) there are two lungs, nearly as in 

 the salamander, though the left is still con- 

 siderably smaller than the right. The respi- 

 ratory motion here, as in the amphibia, is 

 unassisted by a diaphragm, and is principally 

 effected by the ribs and abdominal muscles. 



In Saurian reptiles the respiratory organs 

 are generally formed pretty nearly as in 

 tortoises. The larynx is tolerably simple, 

 without vocal ligaments, and in the chame- 

 leon is furnished with a small sac-shaped 

 appendage: in most Saurians, e.g. the croco- 

 dile, it opens by a longitudinal fissure; but 

 in the chameleon by a transverse one. This 

 opening is always unconnected, being placed 

 far back, and somewhat covered by the pos- 

 terior edge of the tongue in the crocodile, but 

 in other species lying more forwards. Many 

 of the species belonging to this order have 

 the power of emitting a sound by the volun- 

 tary tension of the rima glottidis, as is known 

 to be particularly the case in the Geckos, 

 where the tongue, which can be thrown 

 back like that of the frog, appears to serve 

 as an epiglottis. In the larynx we already 

 find, particularly in the crocodile, a large 

 pointed anterior cartilaginous lamina as a 

 rudiment of the thyroid cartilage. The tra- 

 chea and bronchi are nearly the same as in 

 tortoises, i.e. composed of almost completely 

 circular cartilaginous rings. In the Gecko the 

 trachea is particularly wide and somewhat 

 flattened. The lungs likewise form double 

 cellular sacs, extending downwards far behind 

 the liver; whilst in the crocodile, on the con- 

 trary, they remain above the liver, and, conse- 

 quently, more in the thorax, resembling very 

 nearly, both in iheir shape and position, the 

 lungs of birds. In the chameleon, the lungs 

 are furnished inferiorly with peculiar finger- 

 shaped appendages. Respiration is effected 

 by the thoracic ribs and their muscles, without 

 the assistance of a diaphragm. 



The Circulation of the Blood. In Tortoises 

 the heart is situated immediately above the 

 liver, and close behind the abdominal scutum : 

 it consists of two auricles and a ventricle, the 

 latter being divided into several communicating 

 cells, and presenting a broad circular de- 

 pression, having likewise strong muscular 

 parietes, and being connected at its inferior 

 obtuse extremity by means of a tendinous 

 ligament to the pericardium, as is the case 



in many fishes. The auricles are extremely 



capacious, either of them being very nearly equal 



Fig. 221. 



Heart of the Tortoise. (After Bojanus.) 

 The ventricle opened in front, the left aorta laid 

 open, and a bristle placed in the pulmonary artery. 



in size to the ventricle : they are divided by 

 a septum, which, however, is perforated in 

 the Testudo scorpioides. The right receives 

 the blood of the body by means of the venae 

 cavae, whilst the oxygenized blood from the 

 pulmonary veins enters the left by a fissure- 

 like valvular orifice. The internal arrangement 

 of the ventricle varies somewhat in different 

 instances ; in some, e. g. the Testudo graeca, 

 it is little more than a simple cavity, rendered 

 irregular by the projecting bundles of fibres of 

 its parietes ; in others, on the contrary, e. g., 

 the T. imbricata, these fibres are so very 

 prominent, and appear to divide the cavity so 

 completely into several cells, that Mery was 

 induced to admit the existence of a ventricle 

 for the pulmonary artery and aorta, in ad- 

 dition to a right and left ventricle. Whether 

 the cavity, however, be simple or compli- 

 cated, the course of the blood through the 

 heart is always such, that the pulmonary blood 

 enters at the left side, is mixed with the blood 

 of the venae cavae rather towards the back 

 part of the heart, and then passes on the 

 right side into the aorta, and anteriorly into 

 the pulmonary arteries. 



The principal arterial trunks form a circle 

 Fig. 222. 



Heart of the Tortoise. (After Boj anus.} 

 The ventricle laid open in front, the pulmonary 

 artery slit up, a bristle placed behind a columna 

 carnea, which forms an imperfect septum. 



round the oesophagus, which we must consider 

 as a repetition of the branchial arteries. The 

 aorta, which in the T. imbricata is furnished 

 with two semilunar valves, arises double from 



x 2 



