FISHES. 29 



The spleen is absent in the Cyclostomes and Lepidosiren alone. 

 It is of a blood-red colour, lies mostly on the right side, close to the 

 stomach or the anterior portion of the intestine, and has sometimes 

 one or more parts distinct from it (accessory spleens). In the perch 

 it is placed as a narrow, elongated mass, within the first convolu- 

 tion of the intestine; in the pike it lies behind the stomach, which 

 it surrounds like a hood. In Squatina two spleens occur, a smaller, 

 elongate and oval, in the neighbourhood of the pancreas, and 

 another circular, placed on the stomach at its lower part. In Lamna 

 and Carcharias there are several spleens distinct from each other. 



The lymph-vessels of fishes were first described by HEWSON 

 and A. MONRO (in 1769 and 1770). According to the researches 

 of FOHMANN, who, some years ago, described and figured them 

 more completely, they consist of a simple membrane, resembling 

 serous membrane, and very smooth on the inside. They have no 

 valves, except where the larger stems empty themselves into the 

 veins. These larger stems fall into veins which may be compared 

 to the subclavian veins. In addition there are still other communi- 

 cations between smaller lymph-vessels and venous branches. The 

 chyle, at least in the rays, is of a grey colour. Neither in the 

 mesentery nor elsewhere are conglomerate or lymphatic glands met 

 with in fishes 1 . 



The venous trunks, which return the blood from the different 

 organs, conduct it to a large venous sinus situated between the peri- 

 cardium and the membrane which supplies the office of diaphragm, 

 and is properly nothing but the anterior part of the peritoneum 

 strengthened by some tendinous filaments. The auricle, on the 

 other hand, lies in the pericardium with the ventricle, and is 

 ordinarily broader than it. At the opening of the auricle into the 

 ventricle valves are situated which prevent the reflux of the blood. 



The heart of fishes is small, and its weight forms ordinarily only 

 4-0-0 , sometimes only K^OO part of the weight of the whole body. 

 It is situated behind and between the gills, in osseous fishes under 

 the head, above the junction of the two clavicles (see above, p. 17). 

 The muscular substance of the heart is of a red colour, although 

 the voluntary muscles, as is well-known, are pale or even white. 



1 FOHMANN, Das Saugadersystem der Wirbelthiere, Heft I. Heidelberg u. Leipzig, 

 1827, fol. 



