338 AMPHIBIA xn. 19- 



of X-ray opaque material, allowing the course of the circulation to 

 be watched. It is stated that by this method it can be shown that 

 blood returning to either auricle reaches all parts of the ventricle and 

 that no separation occurs. Since the blood from the skin returns to 

 the right auricle (Fig. 200) it is not clear that a separation of the 

 streams would be advantageous. It may be that the undivided con- 

 dition of the ventricle in amphibians is a secondary development, 

 perhaps not present in the earlier forms such as *Eogyrimis, which 

 reached a larger size (Foxon, 1955). The spongy walls of the ventricle 

 may allow metabolic exchange since the heart is provided only with 

 very small coronary arteries. 



The venous system (Fig. 200) is based on the same plan as that of 

 Dipnoi. The posterior cardinal veins are replaced early in life by a 

 vena cava inferior. Most of the blood from the hind limbs passes 

 through the renal portal system, but there is an alternative path 

 through pelvic veins and a median anterior abdominal vein, which 

 breaks into capillaries in the liver. 



The blood-pressure is regulated by the extrinsic nerves of the heart, 

 fibres from the vagus tending to slow and from the sympathetic 

 nervus accelerans tending to speed the beat. The latter nerve is a 

 new development, there being no sympathetic innervation of the 

 heart in fishes (the condition in Dipnoi is unknown). The diameter 

 of the arteries throughout the body is also under control from sym- 

 pathetic vasoconstrictor and perhaps also vasodilatator nerves. The 

 arterioles in the web of the foot can be seen to constrict when the 

 medulla oblongata is stimulated. Substances extracted from the 

 posterior lobe of the pituitary and from the adrenal medulla also serve 

 to cause constriction of the arteries and perhaps also of the capillaries. 



There is therefore a complex mechanism for ensuring that the pres- 

 sure of the blood is maintained and the flow directed into the part of 

 the body that requires it for the time being. 



20. Lymphatic system of Amphibia 



The transfer of substances between the cells and the blood-stream is 

 effected in any vertebrate by a transudation through the walls of the 

 capillaries into the tissue fluids. Under the pressure of the heart-beat 

 water and solutes leave the capillaries, passing through their walls, 

 while proteins remain behind. The blood passing into the venous 

 ends of the capillaries therefore has a high colloid osmotic pressure 

 and this serves to suck back fluid from the tissues. In this way a 

 circulation from the capillaries into the spaces around the cells is 



