404 PHVSIOLOGY. 



as a connecting link between digestion and respiration. The juices 

 prepared by the intestinal canal require the addition of oxygen from 

 the air before they can be assimilated with the corporeal mass, and for 

 this purpose they pass through the vessels to the respiratory organ. 

 Hence it appears that insects, from the universal distribution of their 

 respiratory organ, require no conducting of the juices, and it was this 

 consideration which, prior to a motion of the blood being observed in 

 them, that was sought to explain their deficiency of blood-vessels, and the 

 consequent deficiency of a circulation was thus illustrated as imperative. 

 We nevertheless find in insects a regular motion of the juices, as was 

 first discovered by the observations of Carus *, and subsequently con- 

 firmed by Wagner t. From the experiments of both these naturalists, 

 the following general result of the mode of this motion of the juices 

 has been found. 



238. 



The juices prepared by digestion pass through the tunics of the 

 intestine into the free cavity of the abdomen among all the organs 

 there situated. It here presents itself as a clear and somewhat greenish 

 fluid, in which oval or round globules swim, which are likewise 

 transparent, and from ^ to ^ of a line in diameter. This fluid is 

 received by the dorsal vessel, or rather by its posterior portion, which 

 we have described as the heart, and which consists of a series of con- 

 secutive chambers furnished with apertures and valves (117); through 

 these apertures during its distension, and then by means of the con- 

 traction of the same organ, through which also the lateral apertures are 

 closed by means of the valves lying in front of them, it is transmitted 

 from one chamber to the other, and then from the last into the aorta J. 

 The number of the contractions and expansions of the heart within a 

 certain time varies according to the stage of development and the state 

 of the temperature. The several chambers also do not simultaneously 

 contract, but, commencing posteriorly, they proceed successively, so 

 that the last and first frequently expand together, whilst the central 



* Entdeckung ernes Einfacben vom Herzen aus beschleunigten Blutlaufes in den 

 I.arven netzfliiglicher Insekten. Leipz. 1827. 4to. 



f Isis, 1832, p. 320. 



{ We must here remark, that this structure of the heart, ascertained to exist by the 

 observations of Straus, was received and taught by even the earlier physiologists. See 

 Bonnet's Contemplation de la Nature, t. i. 



