438 PHYSIOLOGY. 



passing from the head and proceeding laterally, inclining downwards. 

 Thus also in insects the motion of the blood first proceeds forwards and 

 upwards, and then backwards and downwards. The sexual organs, 

 lastly, lie in front of the intestinal canal, therefore above, immediately 

 beneath the heart. This situation is shown above in the ovaries ; in 

 their rudiments also in the caterpillar it was remarked that they lie 

 above the intestinal canal. The orifice only of the sexual organs differs, 

 as in insects it lies beneath the anus, whereas, according to analogy, it 

 should lie above. Nature appears to have pursued a determinate object 

 in this situation, which agrees with that found in fishes, but which 

 could not be subjected to the twisting of the insect body ; but what 

 this object is remains undiscoverable. This only is evident, that the 

 genitalia in insects lie as much in front of the intestinal canal, calcu- 

 lating from the spinal cord, as in the mammalia and other animals. A 

 second objection, besides that of the altered situation of the sexual 

 aperture, which could be made to this twisting of the insect's body, 

 might be deduced from the situation of the mouth, which does not lie 

 laterally with respect to the spinal cord, but upon it, like the former. 

 But the twisting of the anterior half of the head occasions this, and 

 that such a twisting is actually the case is proved by the co-relative 

 situation of the cerebrum and cerebellum, both of which, in fact, do not 

 lie upon one side, but upon the opposite sides of the oesophagus, namely, 

 the cerebrum above, on the true ventral side, the cerebellum beneath, 

 towards the true back. This wholly irregular relation of the parts can 

 only be explained by a twisting of the anterior portion of the head. 

 The organs placed upon that part of the head naturally participate in 

 it, and thus the mouth came beneath, whereas properly it should lie 

 above, were it in harmony with the entire structure of the insect. But 

 how is such a twisting consistent with the simplicity of the case of the 

 head ? The reply to this question appears difficult, but in fact it is 

 not so when we consider that the head as well as the body consists of 

 vertebrae. Their number is regulated by that of the cerebral ganglia ; 

 in the vertebrata there are three, namely, the cerebellum, the posterior 

 lobe of the cerebrum, and its anterior lobe. This anterior lobe, the 

 ganglion of smell, is deficient in insects, as they have no particular 

 organ of smell, and consequently we find in them only two ganglia, 

 namely, the ganglion of sight and the cerebellum. Thus their skull is 

 divided into two vertebral arches. These are also very distinct in the 

 head of the larva ; the posterior one, in which the cerebellum lies, is 



