VERTEBRATA, 421 
forwards through the same angle, but in the forelimb the 
elbow is bent backwards and the foot forwards, resulting in 
a twisting of the two distal limb-bones (radius and udna). 
Thus-is produced the important movement of fronation. 
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- 
comes constricted into 
Fig. 302.—DEVELOPMENT OF THE 
an anterior ventricle VERTEBRATE HEART. 
and a posterior auricle. Heart. 
It then becomes bent a 
upon itself in an §, —— 
hence theauricle comes 
to lie dorsally, and fin- Ventricle. Pericardium. 
ally anteriorly, to the Conus Arteriosus. 
ventricle. Accessory ? Eo es 
to this two-chambered a 
heart in the fishes are Artery. — Auricle. Sinus Venosus. 
the sinus venosus or 
5 Auricle. Sinus Venosus. 
dilated part of the 
veins opening into the Cc 
auricle, and the conus — Branchial 
arteriosus. or valved Artery: 
portion of the ventral Gans: ia 
aorta leaving the ven- 
tricle. This heart is 
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. 
A, A swelling on ventral vessel. __B, Constriction 
into chambers. C, Twisting into an S. 
* The heart in many cases has a double rudiment in the embryo. 
