INTRODUCTION. 
3 
birds ; two sparrows, for instance, inhale more of it than 
a rabbit, which weighs many times more than they. Their 
lungs and blood are therefore of a lighter red colour, and 
their blood is some degrees warmer than that of the mam- 
malia. Their pulse beats on that account more forcibly, 
and is rather like that of a feverish mammal. The greater 
number of them have a loud and, in proportion to their 
size, an enormous voice, which they make frequent use of 
in gladness and sorrow. As a whole, birds may be called 
rather small than large, rather light than heavy. The 
greater number of them are active, swift fliers, mounting 
high into their element, the air. In this class, we see the 
wings developed in length at the cost of the feet. The 
muscles of the breast are in most of them enormous ; in 
like manner is the crest of the sternum, on which the 
muscles are attached. In this class we first observe the 
most artful construction of nests, and the most melodious 
sounds of voicCo Birds are, according to Oken, Ear-hreast 
— and by my researches. Respiration — Animals'' 
All birds agree in having two feet, two wings, a bill, and 
a body covered with feathers. These characters equally 
apply to the class, from the diminutive humming-bird to the 
gigantic ostrich. In some of them, as in the apteryx, the 
