THE BLOOD AND ITS CIRCULATION, 



645 



This lymphatic system of the vertebrated animals is, however, 

 expressed in technical language, only the differentiated interstiti::! 

 sinuses of the lower animals, which has, in the latter, a share in the 

 venous circulation. Indeed, in the lower ver- 

 tebrates the lymphatic tubes frequentlj^ as- 

 sume the form of large sinuses, and connect 

 with the veins. They are even found in the 

 birds. In the frog four of these sinuses 

 have muscular walls, and rhythmically con- 

 tract. These are known as lymphatic hearts. 



In various parts of the body the lym- 

 phatics form glands, such as the thymus, 

 thyroid gland, and the spleen. 



Fishes have a heart resembling that of 

 the mollusks. It is a double force-pump, 

 consisting of a receiving-chamber (auricle), 

 and a propelling chamber (ventricle), with 

 all the valves necessary to prevent a back- 

 ward flow of the blood. But this heart is 

 respiratory it sends the blood directly to the 

 breathing organs ; consequently, it passes 

 only impure blood. When the blood has 

 traversed the gills and is purified, it passes 

 around the circuit of the body through the 

 systemic and portal capillaries, and back to 

 the heart without any further propulsion. 



The low, worm-like fish, lancelet, or am- 

 phioxus, has no special heart, but a number 

 of contractile bulbs in the veins. The eel 

 has such an auxiliary heart in its tail, while 

 the hag has the circulation aided by the contractility of the portal 

 vein. 



Lepidosiren, one of the mud-fishes, approaches the amphibians in 

 the possession of two auricles ; for, in addition to gills, it has true 

 lungs. The vein conveying the purified blood from the lungs joins 

 the left auricle. 



Fig. 1. Diagram of the CrRciTLA- 

 TiON IN A Fish. ^The portion of 

 the system containing pure blood 

 is black ; the part oohtaining im- 

 pure blood is white.) a. auricle, 

 receiving venous blood from the 

 body ; v, ventricle ; m. buibus ar- 

 teriosus ; n, branchial artery, car- 

 ryint; venous blood to the gills 

 (b,b)\ c, aorta, carrying arteri- 

 alized blood to all parts of the 

 body. 



Fig. 2. Diagram op the Lancelet {Amphioxus). m, mouth, surrounded by cartilaginous cirri ; 

 p, greatlv-dilated pharynx, perforated by ciliated clefts ; i, intestine ; a, anus ; h, blood-sys- 

 tem, with pulsating dilatations ; ch, notochord; n, nervous cord. 



