30 CLASS XIV. 



In the osseous fishes the heart is more elongated, conical, in the 

 rays and sharks broad. Its walls are very thick, and the muscular 

 fibres are arranged in two layers, which sometimes separate from 

 each other after death. Commonly not more than 20 or 30 beats 

 of the heart are counted in fishes in a minute, whilst in birds 100 

 and more occur in the same period. The irritability of the heart 

 continues long after death ; it beats, too, often for hours after it has 

 been removed from the body. 



From the anterior part of the heart arises a single arterial stem. 

 In the osseous fishes it commences with a conical thickening 

 (hulbus); in the Cyclostomes this thickening is absent. At the 

 origin of this arterial stem are semilunar valves, commonly two in 

 number, which prevent the return of the blood into the ventricle 

 after its contraction. In the sturgeons, the Plagiostomes, and, ac- 

 cording to the investigations of Mueller, in Polyjciterus and Lep{~ 

 dosteus also, the muscular substance of the heart is prolonged into 

 a cylindrical part before the origin of the arterial stem, and in that 

 cylindrical appendage (a true elongation of the ventricle) are many 

 valves arranged in three or more longitudinal rows'. 



The continuation of the arterial stem now comes to view from the 

 pericardium, and distributes itself as branchial arteries to the respi- 

 ratory organs. The heart of fishes is thus venous alone, and in 

 a physiological point of view is to be compared to the right 

 ventricle of the human heart, whilst the arterial stem, conducting 

 the venous blood to the gills, agrees in function with the pulmonary 

 artery. But it does not follow from this that the heart of fishes 

 corresponds also, in the view of comparative anatomy, to the right 

 ventricle of birds and mammals. The metamorphoses of fi-ogs and 

 salamanders, and the development of the embryo of the higher 

 matter, must here afford the illustration. This comparison teaches 

 us, that from the heart, originally still undivided, there arise on 

 each side difi'erent arterial arches, and that tlie pulmonary arteries 

 are at first only branches of the hindmost of these arches. 



Thus in fishes the branchial arteries are to be compared to the 

 arterial arches of the embryonal state. But, instead of immediately 

 bending round to form the aorta, they separate into branches and 



^ The work of F. Tiedemann, Anatomie des Fischherzens, mit 4 Kupfert. Landshut, 

 1809, 4to, contains many details on the heart of this class. 



