REN. 



233 



with the veins of the testicles or ovaries, and 

 in this manner forms the inferior vena cava. 



The above is a general description of this 

 peculiar system of veins. With reference to 

 its existence in fishes, it will suffice to add, 

 that in different genera this venous system 

 appears in all its modifications. 



In Reptiles, the kidneys are generally situ- 

 ated very far back, even within the cavity of 

 the pelvis, where a sacrum exists, as in the 

 Chelonian and Saurian orders; and in these 

 tribes they are very partially covered by the 

 peritoneum being firmly imbedded in the 

 sacral region. But in serpents, in conse- 

 quence of the elongated form of the body, and 

 the complete flexibility of every portion of the 

 spine, the kidneys are peculiar both in their 

 position and general structure. The kidneys 

 of an Ophidian are not placed upon the same 

 level, but the right is situated much more 

 anteriorly than the left ; a circumstance which 

 much facilitates the packing of the abdominal 

 viscera, and contributes greatly to ensure the 

 free movements of the vertebral column at 

 this place. For the same reason the kidneys 

 of a serpent are divided into numerous lobes 

 of a compressed reniform shape, placed in a 

 longitudinal series upon the external surface 

 of the ureter, and loosely connected to each 

 other and to the spine by cellular tissue and a 

 fold of the peritoneum.* The kidneys of 

 reptiles, like those of fishes, have no distinction 

 of cortical and medullary substance ; and the 

 urinary tubules pass immediately from the 

 substance of the kidney into the ureter. 



The peculiar venous system described by 

 Prof. Jacobson is found in reptiles under 

 the form of the third modification ; that 

 namely, in which the blood returned by the 

 veins from the back and the posterior part of 

 the body, is divided between the kidneys and 

 the vena portas. The exact distribution of 

 these veins in the substance of the kidney of 

 the Boa has been clearly demonstrated by Mr. 

 Bowman, as will be shown, when we come to 

 speak of the minute anatomy of the kidney. 

 The arterial branches, which are comparatively 

 of small size, are derived from the abdominal 

 aorta. 



In Birds the kidneys are elongated in form, 

 commencing immediately below the lungs, and ^ 

 extending symmetrically on each side of the 

 spine, as far as the termination of the rectum. 

 The posterior surface is moulded into the cavi- 

 ties formed by the bones on which it rests. 

 The ureter proceeds from the anterior aspect, 

 the secreting tubules passing immediately, as in 

 fishes and reptiles, into the ureter, so that 

 there is no pelvic cavity in the organ, nor any 

 division into cortical and medullary substance. 

 The outward form of the kidney is very 

 various, and the surface is divided in different 

 species into a variable number of lobes. Each 

 kidney is invested by a delicate capsule, which 

 extends into the substance of the gland be- 

 tween its lobular divisions. Their texture is 

 much more friable than in mammalia, readily 



* Professor Kymer Jones's Animal Kingdom. 



yielding under the pressure of the finger, to 

 which they give a granular sensation as their 

 substance is torn asunder.* 



The peculiar venous system already referred 

 to is described by Jacobson as existing in 

 birds under the form of the third modifica- 

 tion. But the arrangement of the veins 

 differs from that observed in reptiles in this 

 respect, that the crural vein after giving off' a 

 superior branch to the superior lobe of the 

 kidney, and an inferior to the ischiatic vein, 

 sends a middle branch direct to the vena cava. 

 All the blood, therefore, which in birds returns 

 from the posterior part of the body, is carried 

 partly to the kidneys, partly to the portal 

 vein, and partly, but in small quantity, is con- 

 veyed in a direct manner to the vena cava. 

 There are no regular emulgent arteries in 

 birds, the kidneys deriving their arterial blood 

 from various branches of the abdominal aorta. 



The kidneys of Mammalia present one 

 character which is common to the whole tribe, 

 and by which they are distinguished from 

 the other classes of vertebrata. The cha- 

 racter alluded to consists in a division of the 

 substance of the gland into two portions, a 

 cortical and a medullary, the former being the 

 secreting part and containing, as will be more 

 fully shown hereafter, tubes which are very 

 tortuous, while in the latter the tubes are 

 straight, forming minute excretory ducts, 

 through which the secreted products are con- 

 veyed into the ureter. 



In many genera the kidneys are composed 

 of a number of separate lobules or renules, 

 (fg. 142, a a a), each lobe consisting of a cor- 



Fig. 142. 



of the kidney of a porpoise. 



article AVES by Professor Owen. 



