xvi. 24 R ESPI RAT ION 47 1 



restricted mainly to the marrow in the adult, although it may also be 

 found in the liver and spleen. The white corpuscles are more numerous 

 than in mammals. They include neutrophils laden with crystals, and 

 thrombocytes, as well as the mammalian types. Lymphatic tissue is 

 dispersed rather than aggregated into nodes. There is a pair of lymph 

 hearts in the sacral region of the embryo and these may persist in the 

 adult. There is a high basal metabolic rate and a temperature con- 

 siderably higher than that of mammals, usually about 42 C, reaching 

 nearly 45 ° C in some cases. The means by which this is kept constant 

 in the absence of sweat glands are not known certainly. Heat loss is 

 minimized by the absence of vascularized extremities, the feet being 

 little more than keratin and collagen. The formation of the wing from 

 large avascular surfaces has no doubt been a large part of the secret 

 of the success of birds. 



The air-sacs may serve to conserve heat by providing an air cushion 

 for the viscera, with perhaps the alternative possibility of losing heat 

 in this way, by ventilation, when necessary. There is a system of 

 direct arterio-venous connexions in the feet, and elsewhere. The anas- 

 tomotic regions have powerful muscles, whose contraction closes them 

 and forces the blood through the capillary system. There must be a 

 whole system of nervous pathways for the control of upward and down- 

 ward temperature regulation, evolved independently of that found in 

 mammals. At least one species (the nightjar) is known to hibernate, 

 and certain humming birds, whose small size render heat loss a 

 serious problem, become temporarily poikilothermic at night. 



24. Respiration 



Special arrangements are present to provide the large supply of 

 oxygen necessary for the active metabolism and these are based on the 

 plan found in some reptiles. Beyond the respiratory portion of the 

 lung, which is relatively small, there are membranous air-sacs, which 

 are filled at inspiration and then sweep the used air out of the lungs 

 at expiration, thus avoiding the 'dead space' of unrespired air, which 

 is considerable in mammals. When the bird is at rest the air-sacs con- 

 tain air with a high content of C0 2 , but during periods of activity the 

 abdominal air-sacs fill with fresh air containing little C0 2 ; they then 

 serve not only as a means of ventilating the lungs but also for regula- 

 tion of the body temperature. The exact direction of the air currents 

 passing through the lungs and the different air-sacs is not fully under- 

 stood. 



The larynx of birds is a small structure guarding the entrance to the 



