Page:The American Cyclopædia (1879) Volume I.djvu/469

AMPHIBIA 437 organs. The gills, whether external or inter- nal, are expanded in a leaf-like form, consider- ably divided, though much less so than in fishes ; in the earliest stages they are always external, and remain so permanently in the order amphipneura, but in the higher orders they become very soon internal ; they are sup- ported by cartilaginous or bony arches con- nected with the hyoid bone, which changes its form in accordance with the development of the respiratory organ. On the leaflets of the gills the minute blood vessels run, bringing the venous blood into contact with oxygen, and sending it back purified to the heart; While some amphibia retain their gills through life, with coexistent rudimentary lungs, others lose them entirely as the lungs are developed, so that there probably is a brief period in the life of some of these animals in which they may truly be called amphibious. In the caduci- branchiate genera (or those which lose their gills), the early condition of the lungs is that of a mere sac without any appearance of the cells or pulmonary structure afterward developed ; it becomes, therefore, an interesting question whether the similar rudimentary lungs of the perennibranchiate genera can serve any of the purposes of respiration. From the mechanism of respiration in the higher genera, and the anatomical deficiencies in the accompanying apparatus in the lower, it would be at once seen that these simple sacs could hardly aid the gills, and much less could perform their office in aerating the blood. Eusconi con- cludes that in the proteus, at least, these sacs do not assist in respiration ; and experiments alluded to in the "Proceedings of the Boston Society of Natural History," vol. vi., p. 153, show that the menobranchus perishes in about four hours when removed from the water. In the higher orders, the reception of air into the lungs is effected by a simple act of swallowing ; the air enters the mouth through the nostrils, and, the gullet being closed, is forced into the lungs by the action of the muscles of the hyoid bone. The fish-like structure of the nostrils in the lower genera, and the imperfect condition of their glottis and windpipe, with the perfect development of the gills, show that, like the air bag of fishes, these simple sacs, though they represent rudimentary lungs, are not for the purposes of respiration proper. The brain offers the same gradations, from the fish-like simplicity of that of the tad- pole and lower genera to that of the reptile, in which the hemispheres become enlarged lat- erally, and the spinal cord shorter and thicker at the origins of the nerves of the limbs. In the frogs the eyes are large and prominent, in the salamanders comparatively small, and in the Cecilia scarcely visible ; in all they resem- ble those of fishes in the flattened anterior sur- face, the small quantity of the aqueous humor, and the depth of the crystalline lens ; the eyes of the frog are fully developed, and provided with lids. In the lower genera and in the tad- pole state, the organ of hearing is very imper- fect, consisting of a hollow in the temporal bone,- at the bottom of which is the sac contain- ing the cretaceous body on which the nerve is spread ; there is no drum nor tympanic cavity ; the organ is covered by the skin, without any external communication. In the frog the ear is more complicated, having the drum with its cavity and bones, and communicating with the mouth by a Eustachian tube. The sense of smell must be imperfect in the amphibia ; in the lower forms the nostrils are mere cavities in the front of the head, with no communica- tion with the mouth, as in fishes ; in the higher orders the nose communicates with the mouth, but in them the apparatus for smelling is prob- ably less sensitive than in the lower forms, the nose being more connected with the act of breathing. The sense of taste is probably also very obtuse ; in the frogs and toads the tongue is developed to an extraordinary degree, being long, with the anterior half free, doubled back on itself, and capable of being thrown forward and retracted with great quickness, serving for the seizure of the insects on which they feed, and for conveying them instantly to the back part of the mouth. The amphibia are distin- guished from reptiles by the absence of all scaly covering ; a naked skin characterizes all, from the snake-like csecilia to the fish-like axolotl. The skin of the aquatic genera is soft, smooth, and kept constantly moist by the cutaneous se- cretions ; in the land genera, as frogs and toads, the glands of the skin secrete a thick whitish fluid, which may become acrid and irritating, giving rise to the opinion that these secretions are poisonous ; in the salamander the fluid thus poured out is so abundant, and so suddenly se- creted when the animal is irritated, that it is quite probable that even the effects of fire may be for a short time arrested by it doubtless the origin of the well known fable in regard to this animal. The cuticle is frequently shed ; that of the aquatic genera being washed away in shreds, while -that of the toads is divided along the median line, removed by the contortions of the animal, and finally swallowed at a single gulp. The amphibia breathe by the skin as well as by the lungs and gills ; a frog will live from five to six weeks in a state of complete strangulation, and even after total excision of the lungs ; indeed, the pulmonary respiration is insufficient to support life in these animals with- out the assistance of the skin. The medium through which the blood is oxygenated, wheth- er lungs, gills, or skin, whether in air or iii water, is always a modification of the cutane- ous surface ; and, as in frogs the same surface is capable of performing both aquatic and at- mospheric respiration, it is natural to infer that lungs and gills are identical in structure. The amphibia, like many of the invertebrate ani- mals, have the power of reproducing parts lost by accident or design ; the common water sala- mander will invariably restore the limbs which have been cut off for experiment, and even re-