1934 



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- 

 tu>, 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 wall 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 (q.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 weso- 

 m-p/iros or Woljfian 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 extreme interest as indicating the funda- 



FIG. 1637. 



Neural tube 



Fio. 1636. 



Somite 



Neural tube 



Intermediate mass 

 Ectohlast 

 Parietal mesoblast 



/ L \ 



I Viscer 

 Entoblait 



Body-cavity 

 era) mesoblast 



Anlage of nephric duct 

 I'aiielal mesoblast 



Aorta 



Body-cavity 

 Viscera] nu-soblast 

 Remains of intenm-diaU- mass 



P.if 'ion i if catlv rabbit em- 



ivisioll of nn-soblast into 



somite, intermediate mass, and parietal and vis- 

 ceral layers. X too. 



St-i-tion of slightly older embiyo, show- 

 ing differentiation of duct-anla.m' and mass in 

 which tubules develop. X 100. 



mriit.il |-1. 111 upon uliirli. iii a modified form, the later Wolffian body is developed. Although, so 

 knm\n. existing as a permanent or;.-,, in alone in the hag fishes ( My.viiridir}, as a temporary 

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

 tlii- higher animals, even as an embryonal origan, it remains very rudimentary and transient. 

 Wlirn adequately represented, the pronephros consists of a more or less extensive series of 



