228 THE VITAL FUNCTIONS. 



supply of atmospheric air being conveyed through 

 tubes, called trachece, to every part of the body. 

 This structure will be described afterwards ; and it 

 is mentioned here only for the purpose of pointing 

 out the mutual connexion and dependence which 

 exist between these two functions. 



It is necessary, for the active performance of all 

 the functions, that the blood and the air should be 

 brought into extensive mutual contact. If the blood 

 be stagnant, or flow with a languid and uncertain 

 motion, the air must be quickly and extensively 

 brought to act upon every portion of it. Such is 

 the case with insects ; where the deficiency of organs 

 and powers of circulation is supplied by an elabo- 

 rate apparatus of respiratory vessels: an apparatus 

 peculiarly adapted to facilitate motion through the 

 air, but the necessity of which ceases, when this 

 mode of progression is abandoned, and when a 

 more complete system of blood-vessels, capable of 

 effecting the rapid transference and circulation of 

 the blood, is provided. The air is in that case ad- 

 mitted, not indiscriminately to every part, but to 

 certain organs only ; and the blood is carried to 

 these organs, in order that it may undergo the 

 requisite purification by the chemical action of the 

 air. 



These modifications and adjustments are strik- 

 ingly exemplified in the class of Arachnida. In 

 those tribes of arachnida which respire by means of 

 tracheae, the circulation is nearly as imperfect as it 

 is in insects : but in those which have pulmonary 

 cavities for receiving air, we invariably observe a 

 considerable developement of the vascular system. 



