372 PHYSIOLOGY. 



organs which empty themselves into the intestine, even indeed in 

 front of the chylifying portion of it, namely., those blind append- 

 ages indicated as salivary glands behind the proventriculus. 

 4. In the spiders, secreting organs which resemble the biliary vessels 

 empty themselves into the colon ; and other vessels, which are in 

 close connexion Avith the fatty matter, open into the ilium, and 

 supplant the liver. 



To harmonise if possible both views, which then would be the only 

 true and correct one, we must in the first case ascertain if the liver, 

 considering the organisation of insects, be absolutely necessary to their 

 digestion. We find the liver large and of prominent development in 

 all such animals in which the function of respiration is of diminished 

 importance, especially those mollusca which breathe through branchiae, 

 and the fishes "". If we may thence conclude that animals which 

 respire by means of lungs have a smaller liver, it is evident that insects, 

 as those animals in which the respiration by means of lungs, or rather 

 of pulmonary air-tubes, has attained its highest grade of perfection, 

 must necessarily have the smallest liver of all. This may be caused 

 by, as Carus f has remarked upon a similar occasion, the lungs and 

 liver both separating the same substance, namely, such which contain 

 carbon, by the former from an elastic fluid, and by the latter from a 

 liquid. If, therefore, the lung is so predominant that it is found 

 throughout the body, this separation takes place everywhere, and the 

 liver, which by means of the veins receives the carbonated blood from 

 the different parts of the body, where there is no lungs, is not required 

 to act. The function of the liver as an excretory organ is therefore 

 not requisite in insects, but yet as a secretory organ it is still of 

 importance. Its chief object, viewed thus, is to reduce the acidity of 

 the chyme, by means of the alkaline property of its secretion ; but we 

 have seen that the secretions of the salivary glands, and of the pro- 

 ventriculus, are both alkaline, and that the chyme beyond the pro- 

 ventriculus, or at the end of the duodenum, is perfectly neutral, and 

 requires no addition of alkali to neutralise it ; consequently even for 

 this purpose the function of the liver is not necessary. 



If we have thus shown that insects do not require a liver to promote 



* This reciprocal relation appears to me as confirmed, and worthy of consideration, 

 whereas the meritorious G. R. Treviranus denies it. Biologic, torn. iv. p. 4'20. 

 f Bootomie, p. 538. 



