r68 ELEMENTARY BIOLOGY. [CHAP. 



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 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 

 will shew, in the middle line, a dorsal cavity in which the 

 anterior part of the cerebro-spinal axis, the brain, is con- 

 tained, separated by the solid floor of the skull from a 

 ventral cavity, the mouth. 



A transverse section of the abdomen will shew a dorsal 

 cavity containing the posterior part of the cerebro-spinal 

 axis, the spinal cord, separated by the solid floor of the ver- 

 tebral column from a ventral cavity containing the ali- 

 mentary canal and continuous with that of the mouth. 

 But the backward continuation of the alimentary canal is 

 embraced by the large pleuroperitoneal chamber, of which 

 there is no indication in the head. 



On comparing the transverse section of the abdomen of 

 the Frog with a transverse section of the middle of the body 

 of the Lobster, it will be seen that while the chief nervous 

 centre is on one side of the alimentary canal and the heart 

 on the opposite side in both cases, there is no solid and com- 

 plete partition between the nervous centre of the Lobster 

 and the alimentary canal. Moreover, the face of the body 

 on which the nervous centre lies, is that on which the Lobster 

 naturally rests, while in the Frog it is the reverse. The 

 limbs are turned towards the neural side in the Lobster and 



