524 OF SECRETION. 



the Lungs perform for those which are introduced through the Lymphatic 

 system ; namely, the affording an opportunity for the excretion of superfluous 

 or injurious substances contained in the absorbed fluid, before it enters the 

 general current of the Circulation. There is every reason to believe that the 

 conversion of Chyle into Blood is a slow process, requiring the prolonged 

 influence of the latter fluid upon the former; during this influence many 

 chemical changes take place, which are almost certain to be attended with an 

 extrication of Carbon and Hydrogen, these being the ingredients of which the 

 Chyle contains most, when compared with Blood ; and for the extrication of 

 these, the Lungs and Liver afford ready means. Hence we see why the 

 Lacteal system should terminate in a Venous trunk near the Heart, so that the 

 fluid discharged by it will proceed at once to the Lungs ; and why the Liver, 

 wherever it has a distinct circulation, should receive the blood from the walls 

 of the Intestines.* This view derives interesting confirmation from the ex- 

 periments of Cruveilhier, on the artificial production of purulent deposits by 

 injection of Mercury into the veins. He found that when the mercury was 

 introduced into any part of the general Venous system, abscesses in the Lungs 

 were induced ; each inclosing a globule, the irritation occasioned by which 

 was the cause of the purulent deposit. When the mercury was introduced 

 into one of the Intestinal veins, on the other hand, similar purulent deposits 

 occurred in the Liver. It is well known that abscesses in the Lungs and 

 Liver are very common in sequelae of wounds of the head, and of surgical 

 operations, especially those involving bones ; and there seems good reason to 

 believe that in such cases Pus (or some of its elements, which may act the 

 part of a. ferment in exciting suppuration elsewhere), is actually carried along 

 with the current of blood into the Lungs and Liver ; and that, like the glo- 

 bules of mercury, not being susceptible of elimination by these two great 

 emunctories, it acts as a disturbing cause, and occasions disease of their tissue. 

 The fact that a considerable amount of Copper may be detected in the sub- 

 stance of the Liver, after the prolonged introduction of its salts into the system, 

 seems to add weight to this view of its function. It is yet to be ascertained, 

 however, why some substances should be arrested in this organ, whilst others 

 are allowed to pass. 



III. The Kidneys. Secretion of Urine. 



666. The Kidneys cannot be regarded as inferior in importance to the 

 Liver, when considered merely as excreting organs ; but their functions only 

 consist in separating from the blood certain effete substances, which are to be 

 thrown off from it ; and has no direct connection with any of the nutritive 

 operations concerned in the introduction of aliment into the system. Organs 

 destined to the elaboration of a Urinary secretion may be traced very low 

 down in the Animal scale. Among many of the Mollusca we find a small sac, 

 filled with a semi-fluid secretion which has been shown to contain uric acid, 

 opening into the intestine near its anal orifice. In Insects, we often meet with 

 prolonged tubes, resembling the biliary vessels in form, but terminating in a 

 lower part of the intestinal tube ; in some species these are dilated near their 

 extremity into a receptacle for their secretion, or a urinary bladder. Through- 

 out the Vertebrated classes, they exist in a still more evident form. They are 

 uniformly composed of a congeries of prolonged tubes, subdividing and rami- 

 fying more or less ; which spring from the ureter or efferent duct, and termi- 



* Among the Mollusca, the chyle is absorbed by the mesenteric veins, there being no 

 separate lacteal system. These veins, instead of -returning to the heart through the liver, 

 terminate in the branchial vessels; and the process of depuration is effected by the gills. 

 Their liver is supplied only by the hepatic artery. 



