VERTEBRATA. 



421 



forwards through the same angle, but in the fore-limb the 

 elbow is bent backwards and the foot forwards, resulting in 

 a twisting of the two distal limb-bones (radius and ulna). 

 Thus is produced the important movement of pronation. 

 In a great number of mammals which use their fore-limbs 

 almost entirely for progression the bones are permanently 

 pronated, but in others the radius rotates and allows of 

 supination and pronation at the desire of the animal. 



Blood-vascular system. — The heart arises"^ as a 

 contractile portion of the ventral vessel running forwards to 

 the gills. It soon be- 



Fig. 302. — Development of the 

 Vertebrate Heart. 



Heart. 



Ventricle. 

 Conus Arteriosus. 



Pericardium. 



Auricle. Sinus Venosus. 

 Auricle. Sinus Venosus. 



comes constricted into 

 an anterior ventricle 

 and a posterior auricle. 

 It then becomes bent 

 upon itself in an S, 

 hence the auricle comes 

 to lie dorsally, and fin- 

 ally anteriorly, to the 

 ventricle. Accessory 

 to this two-chambered 

 heart in the fishes are 

 the sinus venosus or 

 dilated part of the 

 veins opening into the 

 auricle, and the conus 

 arteriosus or valved 

 portion of the ventral 

 aorta leaving the ven- . , 



. • 1 rpi • 1 , ♦ A, A swelling on ventral vessel. B, Constriction 



triCle. 1 niS neart is into chambers. C, Twisting into an S. 



entirely systemic. In 



the mud-fishes and Amphibia the auricle becomes divided 

 into two by a median septum, the left auricle receiving 

 blood from the lungs only. In the pigeon and rabbit the 

 ventricle also is divided by a median septum, and then the 

 respiratory and systemic currents are completely divided, 

 the right side of the four-chambered heart acting as a 

 respiratory heart and the left as a systemic. 



Conus. 



Pericardium. 

 Ventricle. 



^ The heart in many cases has a double rudiment in the embryo. 



