EXCRETION. 



151 



liquides. Cette vesicule, par sa vitalite' propre, 

 absorbe une partie du fluide qui 1'entoure; 

 ce fluide est ou de 1'eau presque pure, et alors 

 elle eh est simplement impregnee et lubrifiee ; 

 ou de 1'eau plus ou moins chargee de cette 

 matiere gommeuse, elaboree dans les feuilles, 

 et d'autres matieres alimentaires qui peuvent 

 se trouver portees avec la seve dans les diverses 

 parties. La vesicule qui Va absorbee lui J'ait 

 subir une action determinee d'apres sa propre 

 nature, et cette action modifie les materiaux 

 contenus dans la cellule, de maniere a en faire, 

 ou 1'une des matieres communes que nous 

 avons considerees, ou 1'une des matieres que 

 nous aurons bientot a examiner, telles que 

 les huiles volatiles, les resines, &c. Certains 

 vaisseaux analogues a la nature des cellules 

 jouent le meme role sous ce rapport. Les 

 matieres ainsi localement elaborees peuvent, ou 

 rester dans les cellules ou les vaisseaux qui 

 leur ont donne naissance, ou s'extravaser au 

 dehors et donner lieu, soit a des excretions, soit 

 a des transports des matieres d'une partie a 

 1'autre dutissu."* 



The description given by Dutrochet of the 

 act of secretion as it may almost be detected 

 in the glands of the lower classes of animals, 

 is exactly similar. " Entre les vesicules qui 

 composent le tissu organique des animaux 

 rampent les vaisseaux sanguins,chezles animaux 

 a circulation : ces vesicules sont appliquees 

 sur les parois des vaisseaux ; et il est certain 

 que la cavite des vesicules ne communique 

 point immediatement avec la cavite des vais- 

 seaux, puisque le meme fluide n'existe point 

 dans leurs cavites. Ce fait est tres facile a 

 verifier, en examinant au microscope le tissu 

 d'un organe secretive chez uu mollusque gas- 

 teropode, celui de la foie par example : on 

 voit toutes les vesicules de cet organe remplies 

 par la bile, que Ton distingue a sa couleur, 

 tandisque les vaisseaux sanguins qui cotoient 

 ces vesicules n'ont que la diaphaniete que leur 

 donne 1'etat incolore du sang qui les remplit. 

 Ainsi, les vaisseaux sanguins n 'existent que 

 comme des moyens d'irrigation pour les vesi- 

 cules qu'ils cotoient, et ce n'est peut-etre que 

 par filtration que le fluide sanguin penetre, en 

 si modifiant, jusque dans ces vesicules elemen- 

 taires. Le systeme sanguin, considere dans 

 son entier, forme une cavite sans issue, dans 

 laquelle rien ne peut entrer, et de laquelle rien 

 ne peut sortir, autrement que par filtration."-^ 



Any one who is acquainted with the elabo- 

 rate " Vasorum Lymphaticorum Historia" of 

 Mascagni, will recognize the perfect accordance 

 of this statement with the result of his careful 

 and minute investigation of the structure of the 

 secreting organs in the higher animals. J 



We may consider, then, the act of secretion, 

 '' en derniere analyse," as consisting simply in 



* Physiol. Vegetale, p. 215. 



t L'agent immediat du mouvement vital devoile. 

 &c. p. 192. 



$ It must not be considered as ascertained, that 

 the files or tracks of globules of blood seen under 

 the microscope, and usually called capillaries, 

 have really, in all animals, and all parts of these, 

 vascular coats. It seems pretty certain, that in 



the passage of certain portions of a compound 

 fluid through a thin living membrane, and the 

 exclusion of others ; or, according to the for- 

 tunate expression of Dutrochet, as a chemical 

 filtration. " All that is necessary for any 

 kind of secretion in a living animal," says Mr. 

 Mayo, " is a vascular membrane, and all the 

 arrangements of the glands appear to be merely 

 contrivances for conveniently packing a great 

 extent of such a surface in a small compass." 

 And if we are asked, to what cause we can 

 ascribe this escape of certain matters from the 

 circulating fluid through one portion of mem- 

 brane, and of others through another, we can 

 only answer, in the words of this last author, 

 that it depends on the exercise of certain " vital 

 affinities" peculiar to the living state, and the 

 existence of which will always be an ultimate 

 fact in Physiology, although we may attain to 

 a knowledge of the laws according to which 

 they operate. 



V. One principle may already be laid down, 

 almost with certainty, as to the exercise of these 

 powers in the present instance, viz. that the 

 peculiar matters characterizing the excretions 

 are not actually formed from the blood at the 

 parts where they appear, but only separated 

 from the blood at these parts, their formation, 

 if not actually completed, having been at least 

 considerably advanced, in the blood itself 

 which reaches these parts. Of this we are 

 well assured, chiefly by the following facts. 



1 . The experiments already mentioned, first 

 made by Prevost and Dumas, have proved 

 that within a short time after the extirpation of 

 the kidneys in animals, urea may be detected 

 in the blood, showing clearly that the existence 

 of these glands is not necessary to the forma- 

 tion of this very peculiar excrementitious matter, 

 and giving us reason to conjecture that the 

 office of the kidneys is, not to form the urea, 

 but to attract it out of the blood as fast as it is 

 formed there. The same existence of urea in 

 the blood has been ascertained in the human 

 body, both in cases of diseased kidneys, when 

 the excretion there was much impeded, and in 

 cases of malignant cholera, when the excretion 

 was suppressed. The cases of rapidly fatal jaun- 

 dice already mentioned, where the bile-ducts were 

 pervious and empty, would seem to have been 

 cases where the peculiar matter of the bile has 

 been in like manner formed in the blood, 

 without finding the usual vent at the liver. 

 And it will appear under the head of Respira- 

 tion, particularly from the experiments of Dr. 

 Edwards, and of Collard de Martigny, that 

 there is good reason to believe the carbonic acid 

 of expired air to be formed in the course of the 

 circulation, and only exchanged for oxygen at 

 the lungs. 



2. There are various instances in disease, of 

 substances generally found in the secretions of 

 certain glands only, being deposited in situa- 

 tions quite unusual, and where no texture 

 similar to these glands exists ; e. g. cholesterine, 



many cases they are only lines or membranes, or 

 channels in a solid parenchyma ; but still the obser- 

 vation in the text applies strictly to the escape of 

 any particles of the circulating fluid from them. 



