THE NERVOUS SYSTEM AND ORGANS OF SENSE. 217 
bronchus, but before doing so gives off two large arteries, each 
of which is called an innominate artery. Each of these gives 
off an artery named the subclavian (for the wing), and 
then ascends a short way and divides into the carotid and 
vertebral arteries. The vertebral arteries traverse more or less 
of the canal formed by the transverse processes and pleura- 
pophyses of the cervical vertebre *. The carotids may ascend 
side by side to the base of the skull or may meet and blend into a 
single trunk which bifurcates again before entering the cranium. 
Very often, however, there is but a left carotid, which bifur- 
cates at its summit. These various conditions characterize 
different groups of birds. The aorta then passes backwards 
beneath the spine, and supplies all parts of the body, sending 
two large arteries to each of the pelvic limbs. 
On reaching the base of the skull the carotids enter it, passing 
above the basi-temporals and through the sphenoid into the 
cranial cavity. 
The blood of Birds is hotter than that of any other animals, 
and is of a deep red colour. It is also more richly supplied 
than is that of Mammals with those minute bodies called red 
corpuscles, which it always contains. They are elliptical, flat- 
tened, and nucleated, and average 5,5) of an inch in long 
diameter. 
Tue Nervous System AND ORGANS OF SENSE. 
As the internal skeleton is divisible into an axial and an 
appendicular portion, so the nervous system is divisible into a 
central and a peripheral division. 
The central part of the nervous system is made up of the 
brain and spinal cord, while all the nerves which thence proceed 
constitute its peripheral portion. 
Very little need be here said about either, as such structures 
have been hitherto but little used in the definition and classifi- 
cation of Birds. 
The brain affords a good example of the law that this 
organ requires to be of a certain absolute size independently of 
that of the whole body ; for the brain of extremely small Birds 
is relatively very large. 
* See ante, p. 171. 
