ANATOMY OF THE AMPHIBIA. 



343 



HEART OF TllOG. 



(a) Auricle ;() Ventricle; (ao) Aortic Bulb, (1,2,3) 

 the Aortic Trunks ; (eg) Carotid Gland; (0 Lingual 

 Artery; (c) Carotid Artery, (pc) Pulrao-Cutaneous 

 Artery ; tp> Pulmonary Artery, with ramifications 

 over Lungs. 



occipital, basi-sphenoid, supra-occipital, ali-sphenoid bones, or pre-sphenoid cartilage bone ; and a great 



membrane bone, the pre-sphenoid, covers the base of the skull instead, from the occipital to the 



ethmoid region, as in some Fishes. There are two pre-maxillse, and the maxillae are usually present. 



There are pterygoid bones, and, except in some Frogs, there are palate bones. With regard to 



the hinging of the lower jaw to the skull, there is a membrane bone called temporo-mastoid, which 



extends from the side walls of the skull to the articular head of 



the lower jaw, which has a dentary, splenial, and usually an 



angular bone entering into its composition. There are no sternal 



ribs. The heart has but one ventricle, two auricles, and a long 



swelling to the main artery, which has contractile muscular 



fibres, and sometimes valves and longitudinal partitions in it 



called the arterial bulb (bulbus arteriosus). But besides these 



there is a space bounded by contractile walls, into which the 



venous blood from the body pours, before it enters the right 



auricle. The left auricle is usually smaller than the right, and 



a single pulmonary vein from the lungs enters into it. The 



interior of the ventricle is more like a sponge (Huxley) than a 



chamber with well-defined walls. The bulbus arteriosus springs 



from it, and ends on each side in either three or four trunks, 



which ascend upon the branchial arches. The variation in the 



number of these main trunks, which resemble so many arches 



in the different groups of Amphibia, is considerable. In the 



adult and perfect Amphibia the circulation of the blood closely 



resembles that observed in the lowest or simplest amongst the 



Reptilia. In a general sense it may be stated that the little 



left auricle receives the blood from the lungs, whei-e it has been aerated and made to resemble bright 



arterial blood during the pi'ocess of respiration, by means of the pulmonary veins. At the same time 



the larger right auricle receives the dark impure blood from the great veins (vence cavce) of the body r 



head, and internal organs. The auricles thus diversely filled contract, and the pure and impure blood 



are forced into the single ventricle. The mixture is then partly expelled upwards into the muscular 



main artery to supply the body, and partly through a vessel into the lungs, to be re-aerated. The blood 



is cold, and the red blood-corpuscles are large. 



The termination of the main artery into three or four arched trunks on either side relates to the 

 method of respiration of the Amphibia. In some there are lungs within the body, the Frog being the 

 common example ; and in others there are external gills, or branchiae, which last during the whole life 

 of the animal, and the Siren is a well-known type. It is evident that a different distribution of the 

 blood-vessels must occur in them. Moreover, in the Frog and others, like the Tritons, the young 

 immature creature has gills and leads a fish-like life, but the adult has lungs only, so that a very 

 considerable change in the organs of the circulation must occur in them during early life. Again, the 

 gills are external in some, but in others they become internal, and the leaf-like gill seen outside is 

 replaced by internal ones, which are supported by arched bony or gristly processes connected with 

 the hyoicl bone of the throat, and to which the water gains entry through clefts in the side of the 

 neck. Each external gill and each of the internal kind supported on an arch of the hyoid bone, 

 requires a branch of the main artery. When the branchiae are in full action in the Tadpole, and the 

 lungs are still rudimentary, the tympanic bone is greatly enlarged, and it forms the base on which the 

 branchial apparatus is suspended by means of a thick angular portion. Between these angular 

 portions and the median bone is a single piece, and two rhomboidal masses, to which are suspended 

 the arches on which the branchiae are supported, are attached to it behind, one on each side. 



During growth the mandible, or lower jaw, increases in size, the tympanic bone relatively dimi- 

 nishes, the angular portions elongate, and a process of each, cartilaginous in its texture, is attached to 

 the cranium. As the lung is developed and the branchiae are absorbed, the separate parts of the 

 apparatus behind the jaw are gradually fused in one, and the arches are lost. 



In their nervous system the Amphibia are intermediate between the Reptiles and Fish; and if 



