18 INSECTS. 



terious vehicles of communication between mind and body 

 nerves of motion carrying the orders from the mind to 

 the body, nerves of sensation carrying information from 

 the body to the mind, with much more of the abstrusest 

 nature. In all this the brain appears to be the centre 

 of life, and if communication be cut off between the 

 brain and any member, that member becomes useless. 



The nutrition, or building up of the body of an animal 

 (vertebrate or invertebrate) is a compound operation, con- 

 sisting first, of the collection of material ; secondly, of 

 its preparation ; thirdly, of its application ; and fourthly, 

 of its reparation when deteriorated. The collection of 

 material is simply the process of feeding. Its prepa- 

 ration is that of digestion, by which is elaborated out of 

 various substances a fluid containing the ingredients 

 necessary to the nourishment of the body. The third 

 and fourth processes, namely, the application and repa- 

 ration of this building material, the blood, are dependent 

 on circulation and respiration. 



In the vertebrata these take place as follows : 

 The blood, elaborated by digestion from the food, is 

 committed to the heart, which first sends it to the lungs 

 to take in a supply of oxygen from the air which they 

 contain, and then receives it back again to send it forth 

 on its journey through the body. This it performs, at 

 first through the channel of the arteries, visiting every 

 part, and in each depositing some of its constituents. 

 One organ robs a portion of it of one substance, another 

 of another ; each organ, attending to its own business, 

 chooses, probably through the operation of its nerves, 

 its required material. Secretions are formed, tissues are 

 constructed, bones, muscles, fat, receive their appropriate 

 food ; laboratories unnumbered, working without cessa- 



