86 EVERS'S COMPARATIVE ANATOMY. 



CHAPTER XIV. 



SANGUIFEROUS SYSTEM IN THE VERTEBRATA. 



In the greater number of the invertebrate animals which we have 

 hitherto examined, the heart and principal artery were placed above 

 the alimentary canal and the chief part of the nervous system ; but 

 in the vertebrate classes the converse order obtains, the heart 

 being below the alimentary tract; in the former division the blood 

 generally arrives at the heart after having passed through the re- 

 spiratory organ, while in the latter the blood flows from the heart 

 to the respiratory organ. In the invertebrate classes, too, there is 

 no vena porta, the liver being supplied by an hepatic artery alone. 



PISCES. 



The gills, which are the only respiratory organs of fishes, are 

 placed in the course of the arterial circulation. The venous blood 

 from all parts of the body is conducted to a single auricle which 

 propels it into the ventricle, from which it is brought by the 

 arterial bulb to the gills, where it is arterialised, and from which it 

 is distributed by the branches of the aorta to all parts of the body. 

 The caudal vein of the eel presents a contractile dilatation, to which 

 Dr. Hall has applied the name of caudal heart; this doubtless 

 assists in promoting the circulation in the caudal branches of the 

 vena cava. Many look upon the heart of fishes and the artery 

 issuing from it as analogous to the right heart and pulmonary 

 artery of higher animals ; but it is much more just to consider the 

 heart as corresponding to the whole heart of the warm-blooded 

 vertebrata, seeing that in some of the reptiles possessing gills, the 

 blood is sent to these organs through the great systemic trunk. In 

 fact, the heart, in these animals, acts at once the part of a pulmo- 

 nary and a systemic heart in propelling the blood not only to the 

 gills, but through all parts of the aortic system. 



Portal-circulation. — The porta in fishes carries to the liver the 

 venous blood from the stomach, intestines, spleen, pancreas, and 

 occasionally from the genital organs, swimming bladder, and tail. 

 In the gadus, however, as in reptiles, the venous blood from the tail 

 and the central parts of the abdomen goes to the kidneys. In the 

 silurus the blood from the posterior part of the body is distributed 

 to the liver and kidneys : and in the carp, pike and perch, to these 

 organs and to the vena cava. This vessel also receives the blood 

 from the testicle, ovary, kidneys, and frequently from the swimming 

 bladder. 



AMPHIBIA. 



The metamorphosis which this class of animals undergoes in 

 passing from the pisciform to the reptilian state, is strikingly 



