BIRDS 147 



and lymph systems are present, but we need only consider a 

 few points concerning the former. 



The blood is much hotter than that of a mammal, maintaining 

 a temperature of about 103 F. on the average as against 98 F., 

 a fact which has relation to the extreme activity of birds, and 

 the intensity, so to speak, of all their life processes. The well- 

 developed investment of feathers by which the body is covered, 

 and which entangles a large amount of air, is of great impor- 

 tance as a non-conducting coat, which prevents the too rapid 

 dissipation of the heat of the body without checking ventilation 

 of the body surface. A drop of pigeon's blood examined under 

 the microscope shows the same constituents as are seen in a 

 mammal (see p. 39), i.e. a liquid plasma in which are sus- 

 pended white and red corpuscles. The latter, however, though 

 discs, are of a pointed oval shape instead of being circular, and 

 each of them encloses a firmer particle or nucleus. 



The pigeon's heart and blood-vessels agree in essential respects 

 with those of a mammal (see p. 39), but there are numerous 

 differences in detail. Thus, though the heart is four-chambered 

 and the impure blood of the right side is thus completely sepa- 

 rated from the pure blood of the left side, there are differences 

 as regards the valves, especially the one between right auricle 

 and ventricle, which is a muscular flap instead of consisting of 

 three membranous pieces. The great artery of the body, the 

 aorta, where it arises from the heart, curves round to the right 

 and not to the left. The enormously large muscles of flight, 

 which make up the flesh of the breast, are provided with large 

 blood-vessels. 



Breathing Organs and Organs of Voice (fig. 102). The 

 separation between food-tract and breathing-tract is less com- 

 plete here than in a mammal (see p. 34), for the hinder 

 openings of the nasal cavities (posterior nares] are situated on 

 the roof of the mouth instead of farther back. Upon the floor 

 of the pharynx the slit-like glottis is situated, but this is not 

 guarded in front by an epiglottis. It leads into the larynx, which 

 is not in birds the organ of voice, and is continued into the 

 very long windpipe (trachea) that divides into two branches 

 (bronchi] for the lungs. These air -passages are not supported 

 by hoops of cartilage, as in mammals (p. 46), but by bony rings. 

 From this point onwards a number of striking peculiarities are 



