HEART 



401 



reaches a higher stage of development, mid- way between that seen 

 in Elasmobranchs and in Amphibians, and nearly resembling that 

 of Urodeles. The atrium becomes divided into a left and right 

 chamber by a septum : this is also true of the ventricle to some extent, 

 owing to the presence of a cushion composed of muscular fibres 

 and tibro-cartilage arising from the margin of the sinu-atrial aper- 

 ture, extending into the atrium and ventricle, and acting as a valve, 

 ordinary atrioventricular valves being absent. The sinus venosus, 

 from the Dipnoi onwards, opens into the right atrium. 



The conus arteriosus is twisted spirally on itself: in Ceratodus 

 it is provided with eight transverse rows of valves, and begins to 



A 



Fi;. 303. DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH THE HEARTS OF 

 VARIOUS FISHES. (From Boas' 's Zoology.) A, Fish with well-developed 

 conus arteriosus (e.g. Elasmobranch) ; B, Amia ; (7, Teleost. In B and C 

 the sinus venosus and atrium are not indicated. 



<.i. atrium ; b, bulbus arteriosus; c, conus arteriosus ; k, valves ; .<?, sinus venosus ; 

 ./, ventral aorta ; r, ventricle, 



be divided into two chambers. In Protopterus this division is 

 complete, so that two currents of blood, mainly arterial and 

 mainly venous respectively, pass out from the heart side by side. 

 The former comes from the pulmonary vein, from which it passes 

 into the left atrium, thence into the left portion of the ventricle, and 

 thence to the two anterior branchial arteries. The venous current, 

 on the other hand, passes from the right portion of the ventricle into 

 the third and fourth afferent branchial arteries and thence to the 

 corresponding gills, where it becomes purified ; it reaches the aortic 

 roots by means of the efferent branchial arteries. The paired put- 

 to D 



