692 SPECIAL PHYSIOLOGY. 



ing trunks, which unite to form the abdominal aorta. Here, also, a more per- 

 fectly oxygenated blood is provided for the anterior, or more important part 

 of the animal, a less perfectly oxygenated blood, for the hinder part. 



A portal circulation exists in both Reptiles and Amphibia ; it is connected 

 with the renal veins, which also exhibit a renal portal system ; i. e., a set of 

 veins, which convey blood into the kidneys for distribution in their interior. 

 The condition of the organs of circulation in the young or immature Amphibia, 

 will be best understood after that of Fishes has been described. 



The cold-blooded branchiated Vertebrata, or Fishes, have no lungs ; those or- 

 gans being represented, however, in a few species, by the so-called air-bladder, 

 an appendage, usually of the pharyngeal part of the alimentary canal. The 

 heart is now simplified by the suppression of the left auricle, there being no 

 pulmonary veins to end in it ; the ventricle also is single, as in the Amphibia 

 and Reptiles, but it never presents any trace of an internal septum. There 

 remain, therefore, but a single auricle and a single ventricle. The auricle, 

 like the right auricle in other cases, receives the dark venous blood from the 

 body, and transmits it, through an orifice provided with valves, into the ven- 

 tricle. This is very muscular, and propels the blood, not into the lun^s, for 

 there are none, nor directly into a systemic arterial trunk, or aorta, for imme- 

 diate distribution through the body generally ; but, on the contrary, the single 

 ventricle sends it into a short trunk, called the arterial bulb, which has valves 

 at its root, and, in a few examples, is even partially divided into two. From 

 this bulb, a series of arched vessels, usually five, sometimes four, in number, 

 proceed upwards, supported on the cartilaginous branchial arches, and convey 

 the blood, through the branchial arteries, into the gills, in which it passes 

 through capillary vessels, and is then collected by the branchial veins ; these 

 running towards the vertebral column, unite together an the dorsal aspect of 

 the alimentary canal, to form a single systemic arterial trunk, which corre- 

 sponds, in function, with the aorta of the higher Vertebrata. From this, 

 branches are given off to all parts of the body, excepting 'the gills and gener- 

 ally, if not always, certain parts of the head, which are, singularly enough, 

 supplied by special branches proceeding at once from the branchial veins. 

 From the body, generally, the blood is brought back, by the systemic veins, 

 to the single auricle. In the Fish, therefore, all the blood which returns from 

 the body to the heart, is sent first through the respiratory or oxygenating or- 

 gans, before it re-enters the system ; the respiratory apparatus is so interposed 

 that it forms a part of the general circulation, the branchial circulation being 

 continued on to the systemic circulation, without the blood coming back to the 

 heart between them. Hence the circulation in the branchiated Fish, is said 

 to be single, as distinguished from the imperfectly double circulation of the cold- 

 blooded, and the completely double circulation of the warm-blooded, pulmonated 

 Vertebrata. The heart of the Fish is also said to be a branchial and not a 

 systemic heart, because its immediate work is to force the blood into the 

 branchiae or gills. No special contractile apparatus exists beyond the gills, to 

 accomplish the circulation through, the body a fact which has already been 

 adduced (p. 674), to show that the heart's action is adequate to drive the blood 

 through the systemic circulation of Man, and the pulmonated Mammalia. In 

 the caudal veins of the eel, however, as already mentioned, there exists a pul- 

 sating portion, named a venous heart, which undoubtedly assists in the return 

 of the venous blood to the distant proper heart. Its presence may be connected 

 with the unusual length of the systemic vessels in this part of the eel ; but 

 even here, the heart propels the blood through the vessels of the gills, and af- 

 terwards through the systemic arteries and capillaries. Pulsating dilatations 

 are likewise .found in the arteries of the pectoral fins of the torpedo and chi- 

 msera, and in the portal vein of the myxine. 



In Fishes, as in other Vertebrata, a portal system of veins exists ; it is com- 

 posed not only of the veins from the digestive organs, but also of those from 

 the other viscera in the posterior part of the abdomen, and likewise of some of 

 those from the hinder part of the body. The venous trunks thus formed, con- 

 duct blood to the kidneys as well as to the liver, so that, in Fishes, both these 

 f lands receive venous blood. After being distributed through them, the blood 

 returned to the heart, by proper hepatic and renal veins, which open into 



