160 ELEMENTARY BIOLOGY. [XIII. 



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 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 abdominal 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 suddenly 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 alimentary canal. 



Separated from the pleuroperitoneal and oral cavities by 

 the bodies of the vertebrae and the hard roof of the oral 

 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 

 membrane is reflected on to 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. 



A transverse section of the head in the region of the eyes 



