REN. 



251 



plexus ; which they would fail to do if they 

 entered it in any other part." 



" I have described the renal artery as being 

 spent upon the Malpighian bodies ; but in 

 the hilum of the lobe it gives off, as in the 

 higher animals, a few slender twigs to the 

 coats of the excretory ducts, and of the 

 larger vessels. The capillaries of these twigs 

 are easily seen, and, in all probability, dis- 

 charge themselves into the branches of the 

 portal vein." 



It will appear on referring to the plan {fig. 

 161.), that there is a direct relation between 

 the size of the Malpighian bodies and the 

 width of the lobe. At the apex of the 

 lobe, where the uriniferous tubes are com- 

 paratively short, the Malpighian bodies are of 

 small size, while at the base of the lobe, where 

 the tubes are longer, the Malpighian bodies 

 present a corresponding increase of size. It 

 will presently be seen that this and other facts 

 in the anatomy of this form of kidney, afford 

 very important evidence as to the nature and 

 office of the Malpighian bodies. 



Mr. Bowman thus draws a comparison be- 

 tween the circulation through the kidney of 

 the Boa and that through the liver : " The 

 circulation through this form of kidney may 

 be aptly compared with that through the liver, 

 as described by Mr. Kiernan in his invaluable 

 paper on that gland. The plexus surrounding 

 the tubes corresponds with the portal-hepatic 

 plexus, which, in the lobules of the liver,invests 

 the terminal portions of the bile-ducts. Both 

 these plexuses are supplied with blood by a 

 portal vein, derived chiefly from the capillaries 

 of distant organs, but in part from those of 

 the artery of the respective organs them- 

 selves. The only difference seems to be, 

 that, while in the liver the branches of the 

 artery are entirely given to the larger blood- 

 vessels, ducts, &c., in the kidney a few only 

 are so distributed, the greater number go- 

 ing through the Malpighian bodies, to per- 

 form an important and peculiar function. In 

 both glands, however, all the blood of the 

 artery eventually joins that of the portal vein. 

 The emulgent vein of the kidney answers to 

 the hepatic vein of the liver." 



" Tlie comparison between the hepatic and the 

 renal portal circulation may be thus drawn in 

 more general terms. The portal system of 

 the liver has a double source, one extraneous, 

 the other in the organ itself ; so the portal 

 system of the kidney, in the lower tribes, has 

 a two-fold origin, one extraneous, the other 

 in the organ itself. In both cases the extra- 

 neous source is the principal one, and the 

 artery furnishing the internal source is very 

 small. But in the kidney of the higher tribes 

 the portal system has only one internal source, 

 and the artery supplying it is proportionably 

 large." 



Mr. Bowman has ascertained that in all the 

 vertebrate classes the Malpighian bodies have 

 essentially the same structure ; the capsule 

 being formed by the dilated extremity of a 

 uriniferous tube, into which a single mass of 

 blood-vessels is inserted. But in some orders 



of animals there are modifications which merit 

 notice. The most considerable of these re- 

 gard the size of the Malpighian bodies. The 

 following table from Mr. Bowman's paper ex- 

 hibits their size in a few species, and subjoined 

 to each measurement is that of the tube soon 

 after its emergence. It will be seen that the 

 diameter of the tubes varies far less than that 

 of the Malpighian bodies. 



Table of the Diameter of Malpighian Bodies, and of 

 the Tubes emerging from them, in fractions of an 

 English inch. 



According to Professor Miiller * the kid- 

 ney of the myxinoid fishes has a very 

 simple structure. Before the publication of 

 Mr. Bowman's paper Miiller described the 

 kidney of these fishes, as consisting of a long 

 ureter extending on each side of the intestine, 

 and sending off at intervals a small sac which 

 terminates in a second closed sac, the junction 

 of the two sacs being marked by a constric- 

 tion. In the cavity of the closed sac there is 

 a globular tuft of vessels, which is free on all 

 sides except at one point, where the vessels 

 pierce the investing capsule (fig. 162.). Prof. 

 Miiller, from a comparison of his own obser- 

 vations with those of Mr. Bowman, infers that 

 the short tubes proceeding from the ureter in 

 these fishes are analogous to the uriniferous 

 tubes in the more highly organised kidneys, 

 while the closed sac at the extremity of the 

 tube is analogous to the Malpighian capsule ; 

 so that each renculus in the myxinoid fish 

 consists of an exceedingly short uriniferous 

 tube terminating in a capsule, in which is sus- 

 pended a globular tuft of vessels. The arte- 

 rial branches which come directly from the 

 aorta terminate, as in the higher animals, by 

 piercing the capsule and forming a globular 

 tuft within it. Miiller had not an opportu- 

 nity of tracing the exact distribution of the 



* Untersuchungen uber die Eingeweide der Fische. 

 Berlin, 1845. 



