10 ELEMENTARY BIOLOGY. [CHAP. 



of the lower jaw. In a state of rest, therefore, it lies on the 

 floor of the mouth with its free end turned backwards, and 

 one point on each side of the glottis. 



The gullet, after traversing the so-called diaphragm, 

 passes into the elongated stomach. At its posterior end this 

 narrows and joins the slender small intestine. Though short, 

 this is too long relatively to the length of the pleuroperi- 

 toneal cavity to lie straight in it. It is, therefore, thrown 

 into sundry folds which are suspended to the dorsal wall of 

 that cavity in the manner before described. Finally, the 

 small intestine enters the dilated short large intestine, and 

 this opens into a chamber with muscular walls, the cloaca, 

 the external aperture of which has been already mentioned. 



Thus the alimentary canal is a tube which traverses the 

 body from the oral to the anal apertures ; and the heart, en- 

 closed in the pericardium, is situated in the middle line on 

 the ventral side of the same. 



Separated from the pleuroperitoneal and buccal cavities by 

 the bodies of the vertebrse and the hard roof of the buccal 

 chamber which continues the direction of these forwards, is 

 an elongated cavity, widest in the head but becoming very 

 narrow posteriorly, which is closed on all sides by the bony 

 and other elements of the head and spinal column. This is 

 the neural cavity and contains the brain and spinal cord, 

 which together constitute the cerebro-spinal nervous axis. The 

 neural cavity is lined by a serous membrane resembling the 

 peritoneum and the pericardium, and this arachnoid mem- 

 brane is reflected on fo and covers the contained cerebro- 

 spinal axis, so that the latter is related to it as the heart is to 

 the pericardial membrane. 



The cerebro-spinal nerves which are given off from the 

 brain and spinal cord pass to their destination through the 

 boundary walls of the neural cavity. 



