ORGANS OF SENSATION. 75 



The air then is breathed by means of these various 

 organs, and acts on the blood, or the fluid similar to 

 blood, somewhat, it may be presumed, in the same way 

 as it acts on the human blood, in the lungs, oxygen 

 being abstracted, and carbonaceous matter carried 



off('). 



The insects which live under water constantly or 

 partially, have peculiar organs for decomposing the 

 water or the air it contains in order to procure oxygen, 

 which appears to be indispensable to life. Some of 

 these water insects, indeed, come ever and anon to the 

 surface, in the same manner as the water eft and the 

 whale, to breathe the air. Others remain always un- 

 der water. 



ORGANS OF SENSATION. 



IT is well known that in man, the brain and spinal 

 cord, with the nerves proceeding from them or con- 

 nected with them, are the organs of feeling or sensa- 

 sation, the brain being encased in the strong bones of 

 the scull, and the spinal cord in the no less strong 

 and peculiarly-jointed bones of the back. The nerves 

 in insects differ much from those of man, and particu- 

 larly in there being no peculiar structure like the 

 skull, and the bones of the spine, to encase what may 

 be termed the main stem, whether that be considered 

 the origin or the receiver of the branches. This main 

 stem in insects, however, which lies along the breast 

 and belly, from the head to the vent, is protected by 

 the inner breast-plate from being compressed by the 

 gullet, as has been already noticed. 



In man, the brain is distinguished into two parts, 

 the spinal cord being a third ; but besides these there 

 is a system of numerous and extensive nerves, which 



(1) This will be fully explained in the "ALPHABET OP PHYSI! 



OLOGY." 



