282 LECTURE XI. 



RENAL SYSTEM. 



In all Vertebrates an excretory organ is very early developed in 

 the form of a tube, extending from each side of the cloaca forwards, 

 along the dorsal region of the abdomen, close to the spine, where it 

 communicates with a number of short slender blind tubes placed at 

 right angles to it ; the longitudinal trunk-tube serving as the ex- 

 cretory duct of the shorter transverse secerning ca^ca. These glands 

 are transitory in the air-breathing Vertebrates, and are called, from 

 their discoverer, ' corpora Wolffiana ;' they are persistent in fishes*; 

 and are called ' kidneys : ' in both they are renal organs and secrete 

 urine. A slightly opaque, slender, elongated glandular body, in the 

 situation mai'ked li in jig. 46., has appeared to me to rej)resent the 

 renal organ in the Branchiostoma. The structure of this organ is 

 more obvious in the Myxinoids : it is double : each long duct, as it 

 extends from the cloaca through the abdominal cavity, sends off, at 

 regular but distant intervals from its outer side, a short wide tube, 

 which communicates by a narrow opening with a blind sac. At the 

 bottom of this sac or ciBCum there is a small vaso-ganglion, free on 

 all sides save that by which the vessels enter and quit it j : there are 

 no uriniferous tubes in this ' placentula ; ' the contents of the caecum 

 must react through its thin parietes, and those of the capillaries with 

 which it is in contact, upon the blood in those capillaries, and ex- 

 tract therefrom the azotized uric excretion. Analogous vascular 

 bodies, formed chiefly by convoluted tufts of artei'ial capillaries, are 

 present in the Wolffian bodies of Mammals, and in the persistent 

 renal organs of all Vertebrates. They are called, after their discoverer, 

 ' Malpighian corpuscles,' and Mr. Bowman | has admirably shown 

 how the uriniferous tubes take their rise from these vascular corpus- 

 cles, viz. by a sacciform blind beginning applied over the vascular 

 placentule or tuft. 



The combined secerning ca^ca and vaso-ganglia form in the Sand- 

 prides and Lampreys § a continuous narrow elongated gland, which 

 extends in the former (^A?nmocetes) throughout the abdomen, in 

 the latter (^Petromyzon) along the posterior two-thirds : in both con- 

 fined to the dorsal part of the cavity. The ureters {fig. 74. k) open 

 into the short canal (ib. /), leading to the papillary production of 

 the peritoneal outlets close to the anus. || 



* cxxxvii. ii. p. 314. f xxi. 1841, p. 13. \ cxxv. 



§ In the Pelromyzon marinus the diameter of the tuhuli uriniferi is 5g:jth of an 

 inch, that of the capillaries of the kidneys being yjjjgth of an inch. 

 II XX. iv. pi. 56. fig. 1. e. 



