224 THE VITAL FUNCTIONS. 



perish when placed in the receiver of an air-pump, and the 

 air exhausted; but they are generally more tenacious of life 

 under these circumstances than the larger animals, and often, 

 after being apparently dead, revive on the readmission of 

 air. 



Aquatic insects have tracheae, like those living in air, and 

 are frequently provided with tubes, which are of sufficient 

 length to reach the surface of the water, where they absorb 

 air for respiration. In a few tribes a complicated mode of 

 respiration is practised; aerated water is taken into the body, 

 and introducecj into cavities, when the air is extracted from 

 it, and transmitted by the ordinary trachea to the different 

 parts of the system.* 



Such, then, is the extensive apparatus for aeration in ani- 

 mals, which have either no circulation of their nutritious 

 juices, or a very imperfect one; but no sooner do we arrive 

 at the examination of animals possessing an enlarged sys- 

 tem of blood vessels, than we find nature abandoning the 

 system of tracheae, and employing more simple means of 

 effecting the aeration of the blood. Advantage is taken of 

 the facility afforded by the blood vessels of transmitting the 

 blood to particular organs, wnere it may conveniently re- 

 ceive the influence of the air* Thus, Scorpions are provided, 

 on each side of the thorax, with four pulmonary cavities, 

 seen at L, on the left side of Fig. 374, into each of which 

 air is admitted by a separate external opening. A, B, is the 

 dorsal vessel, which is connected with the pulmonary cavi- 

 ties by means of two sets of muscles, the one set (M, M) be- 

 ing longer than the other (m, m, m.) The branchial arte- 

 ries (v) are seen ramifying over the inner surface of the 



* Mr. Dutrochet conceives that the principle on which this operation is 

 conducted is the same with that by which gases are reciprocally transmitted 

 through moistened membranes; as in the experiments of Humboldt and Gay 

 Lussac, who, on enclosing mixtures of oxygen, nitrogen, and carbonic acid 

 gases, in any proportion, in a membranous bladder, which was then im- 

 mersed in aerated water, found that there is a reciprocal transit of the gases 5 

 until at length pure atmospheric air remains in the cavity of the bladder. 



