THE NBETOUS SYSTEM AND OEGANS OF SENSE. 2 1 7 



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 vertebrse*. 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 

 corpmcles, which it always contains; They are elliptical, flat- 

 tened, and nucleated, and average -j^Vo '•^ ^^ '^^'^^ ^^ -^o"? 

 diameter. 



The NEETors System and Oegans or 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. 



Theii' hrain 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. 



