AVES. 245 



present. The three chief digestive juices are, gastric juice, bile, 

 and pancreatic juice, secreted respectively by the peptic glands, 

 liver, and pancreas, and acting as in the frog. An important 

 addition to the absorptive surface is afforded by the villi. 



5. Circulatory Organs. Blood and lymph systems are present, 

 and well developed. 



I. Blood System. The bright red blood, which coagulates 

 very quickly, is made up of plasma, with amoeboid colourless 

 corpuscles and much more numerous red corpuscles. Both kinds 

 are nucleated, and the latter are small, flattened, and oval, with 

 ends somewhat pointed. The blood circulates in a closed system 

 of tubes made up of heart, arteries, veins, and capillaries. 



The large conical heart (Fig. 70), contained in its pericardium, 

 lies in the anterior part of the thorax, and is made up of four 

 chambers, two auricles and two ventricles. Its forwardly directed 

 base is chiefly made up of the dark thin-walled right and left 

 auricles, its apex by the thick-walled right and left ventricles, the 

 latter being the firmer, and alone reaching the extreme apex. The 

 cavities of the auricles are separated by a thin auricular septum, 

 upon which is a depression, the fossa ovalis, where the septum is 

 thinnest. Within the right auricle are the three openings of the 

 caval veins, that of the postcaval being guarded by a muscular flap, 

 the Eustachian valve, and, posteriorly, the crescentic right auriculo- 

 ventricular opening leading into the right ventricle. From the right 

 side of this opening a muscular flap, the right auriculo-ventricular 

 valve, projects into the ventricular cavity, to the walls of which it 

 is united by fibrous cords (chordce tendinece). The right ventricle is 

 separated from the left by the ventricular septum, which bulges 

 into its cavity, the walls of which are raised into muscular ridges 

 (columnce carnece). The pulmonary artery, which arises from this 

 division of the heart, has its origin guarded by three pulmonary 

 semilunar valves (pocket-valves). Within the left auricle, dorsally, 

 is the small opening of the pulmonary veins, and, posteriorly, the 

 rounded left auriculo-ventricular opening. This is guarded by a 

 mitral or bicuspid valve, made up of two membranous flaps, 

 situated in the cavity of the left ventricle, and connected by 

 chordae tendinese to papillary muscles, conical elevations, which 

 may be considered as modified columnse carnese. The aorta 

 arises from the anterior end of the left ventricle, and its origin 

 is guarded by three aortic semilunar valves (pocket- valves). 



