232 
ZOOLOGY. 
more than six. The tail, extremely variable in shape, has 
twelve rectrices (with certain anomalous exceptions). 
There is but one common carotid artery, and the sternum 
is very uniform in sliape. Their high physical irritability 
is co-ordinate with the rapidity of their respiration and 
Fig. 273.— Magpie. 
circulation; they consume the most oxygen and live the 
fastest of all birds (Coues). 
There are two groups of Passerine birds, differing in the 
structure of the lower larynx; in the first {Clamatores) the 
vocal organs are more or less rudimentary, the species not 
being singers, while in the second and higher division 
{Oscines) the lower larynx is so developed that most of the 
species excel as singers. In the singing birds the vocal 
