6 Vertebrata. 



CHAPTER II. 



CRANIOTA. 



4. General characters of head-bearing vertebrates. 



The head-bearing vertebrates are characterised by 

 the enlargement of the anterior end of the central 

 axis of the nervous system into a series of swellings 

 which collectively make up the brain. To contain 

 this brain the fore part of the neural canal is enor- 

 mously dilated, and its walls are converted into a 

 gristly, membranous, or bony case, called the skull, 

 and in this part of the body the organs of sense are 

 chiefly seated. The brain in its simplest form con- 

 sists of three thick-walled cavities in a series from 

 before backwards, the walls of the three being named 

 respectively the fore, mid, and hind brain. The rest 

 of the neural canal behind the skull remains as a 

 narrow tube enclosed in the canal, which is bounded 

 by the neural arches of the vertebrae. In the sides 

 of the wall of the cranial or skull cavity the organs of 

 the special senses are placed in a series from before 

 backwards ; foremost of them is the organ of smell ; 

 secondly, that of sight ; thirdly, that of hearing. Each 

 of these organs consists primarily of a pouch of skin 

 bulging towards the inside of the body, and receiving 

 a nerve from the brain. Passing out from the brain 

 there are also other nerves, which are distributed to the 

 parts of the foremost end of the body. Around these 

 organs and nerves the cartilage which forms the primi- 

 tive skull becomes disposed so as to protect them; and 



