if934 



HUMAN ANATOMY. 



the urethra can then be considered as normally longer than the superior surface. The 

 term "surgical wall," proposed for the upper wall by Guyon, would seem to be 

 merited, because it offers the shortest route to the bladder, is the most regular and 

 constant as to form and direction, presents the smoothest and firmest surface, is the 

 less capable of gliding before an instrument or being modified by mechanical pressure, 

 offers the greatest resistance to rupture and penetration, is less intimately connected 

 with important structures, and is the less vascular of the two walls. As to the calibre 

 and distensibility of the urethra, enough has already been said ; but it should not be 

 forgotten that there are three relatively constricted parts, the internal or vesical mea- 

 tus, the external meatus, and the membranous regions ; and three dilatations, the 

 fossa navicularis, the bulbar cul-de-sac, and the prostatic depression, the last two dila- 

 tations presenting numerous individual variations ; and in this connection it is impor- 

 tant to remark that all three of these dilatations are excavated at the expense of the 

 inferior w^all of the canal. The urethral curve only remaining regular in the superior 

 wall, it results that the more pronounced the curve the more accentuated are the bul- 

 bar and prostatic depressions ; and as a certain degree of lengthening of the urethra 

 always corresponds to the greatest curve, — since these are both produced by bulbar 

 and prostatic augmentation of volume, — one can reasonably conclude that urethrae of 

 the greatest curves present at the same time the greatest length. With a knowledge 

 of these facts, the instrumental exploration of the urethra becomes a matter of much 

 accuracy and precision (Morrow). 



The anatomy of the various forms of urethrotomy and other operations on the 

 urethra is sufficiently dealt with in the foregoing and in the practical considerations 

 relative to the bladder, male perineum, and prostate (g.v.). 



DEVELOPMENT OF THE URINARY ORGANS. 



The development of the essential parts of the urinary tract — the kidney and its 

 duct — is so intimately related with the foetal excretory organ, the Wolffian body, that 

 a brief account of the latter and of the principles underlying its genesis is a necessary 

 introduction to the intelligent consideration of the subject here to be presented. The 

 excretory apparatus of amniotic vertebrates, even in the highest mammals and man, 

 includes three structures which, although as functionating organs existing in no 

 single animal, stand in genealogical sequence. These are the pronephros, the ?neso- 

 nephros or Wolffian body, and the metanephros or definitive kidney. 



The Pronephros.— The first of these, the pronephros, sometimes called the " head-kidney" 

 on account of its anterior position in its primary condition, in all higher forms is at best a rudi- 

 mentary and functionless organ ; nevertheless, it is of e.xtreme interest as indicating the funda- 



FiG. 1636. 



Neural tube Somite 



Intermediate mass 

 ./ Ectoblast 



Parietal mesoblast 



Noto- 

 chord 



Body-cavity 

 Aorta / Visceral mesoblast 

 Entoblast 



Part of transverse section of early rabbit em- 

 bryo, showing primary division of mesoblast into 

 somite, intermediate mass, and parietal and vis- 

 ceral lavers. X 100. 



Fig. 1637. 



Neural tube 



Somite 



Anlage of nephric duct 

 Parietal mesoblast 



Aorta 



Body-cavity 

 Visceral mesoblast 

 Remains of intermediate mass 



Section of slightly older embryo, show- 

 ing diiTerentiation of duct-anlage and mass in 

 which tubules develop. X 100. 



mental plan upon which, in a modified form, the later Wolffian body is developed. Although, so 

 far as known existing as a permanent organ alone in the hag 'a%\\&s[Myxinid(r) as a temporary 

 structure the pronephros attains considerable development in many fishes and amphibians ; in 

 the higher animals, even as an embrvonal organ, it remains very rudimentary and transient 

 When" adequately represented, the pronephros consists of a more or less extensive series ot 



