47° THE BIRDS xvi. 22- 



purpose. The bursa Fabricii is a blind sac with much lymphoid tissue, 

 opening into the proctodaeum; its function is probably to protect 

 locally against infection and to produce lymphocytes for the blood- 

 stream, hence it has been called a 'cloacal thymus'. Like the thymus, 

 it is prominent in young animals and usually much reduced in the 

 adult. 



The large surface area, high temperature, and great activity of birds 

 necessitate a high food intake, especially in the smaller types. This is 

 made possible by rapid passage of food through the gut. Thus a shrike 

 (Lanius) is said to digest a mouse in 3 hours, and hens take only 12-24 

 hours over the most resistant grain. The amount of food taken per 

 day may reach nearly 30 per cent of the body weight (6 g) in the very 

 small goldcrest (Regains) but is about 12 per cent in a starling 

 (Sturnus) weighing 75 g. 



23. Circulatory system 



Many of the features characteristic of birds depend on an efficient 

 circulation, allowing of a high rate of metabolism, and hence a high 

 and constant temperature. It is significant that the birds and mammals 

 are the only vertebrates that have achieved complete separation of the 

 respiratory and systemic circulations, making possible a high arteriolar 

 pressure, which allows materials to reach the tissues rapidly. 



The heart shows its sauropsidan characteristics clearly in that the 

 ventral aorta is split to its base into aortic and pulmonary trunks. The 

 former arising from the left ventricle curls round the pulmonary 

 trunk to form a single right aortic arch. The heart has lost the sinus 

 venosus; as in mammals no such extra chamber is necessary to step 

 up the venous return pressure. The ventricles are large, especially the 

 left. The right auricle and ventricle are separated by a flap-like valve, 

 the left side having valves with chordae tendinae, somewhat as in 

 mammals. There are enormous innominate arteries to supply the 

 pectoral muscles. In the venous system there are renal portal veins. 



The size of the heart and rate of heart-beat vary with the size and 

 activity of the bird, larger birds having in general relatively smaller 

 and less rapid hearts. In a turkey the rate of beat may be less than 100 

 per minute, in a hen about 300, and in a sparrow nearly 500. 



The red corpuscles of birds differ from those of mammals in being 

 oval and nucleated. They carry a large amount of a haemoglobin that 

 gives up its oxygen suddenly at a relatively high oxygen tension. The 

 red corpuscles are smaller in actively flying birds than in the larger 

 flightless ratites. Haemopoetic tissue is widespread in the young, 



