THE BLADDER. 217 



The fibrous coat, or tunica adventitia, composed of bundles of fibro- 

 elastic tissue, invests the entire renal duct as its outermost tunic and connects 

 it, through the surrounding areolar tissue, with the adjacent structures. 

 Within the sinus of the kidney, the outer coat of the renal duct blends with 

 the tunica fibrosa that invests the renal substance where the latter is not 

 embraced within the calyces. Just above the bladder the fibrous coat of the 

 ureter becomes thicker and, in conjunction with the longitudinal muscle, 

 forms the ureteral sheath, giving independence to the duct as it passes 

 through the wall of the bladder. 



The blood-vessels supplying the renal duct, derived from several 

 sources during its long course through the abdomen and pelvis, break up 

 into capillaries which are especially numerous within the tunica propria 

 immediately beneath the epithelium. The veins begin within the mucosa, 

 beneath which they form an inner plexus that communicates with a wider 

 meshed outer plexus, lying within the fibrous coat and giving rise to the 

 larger emergent venous trunks. The lymphatics within the mucous mem- 

 brane are indefinite lymph-channels, but within the muscular coat and on the 

 surface are present as distinct networks, from which afferent vessels pass to 

 various groups of lymph-nodes. The nerves distributed to the renal duct 

 are brought by branches from the neighboring sympathetic plexuses. While 

 consisting chiefly of nonmedullated fibres destined for the muscular tissue 

 and blood-vessels, many sensory fibres find their way into the mucous mem- 

 brane, where some end within the tunica propria in free arborescent endings 

 and others between the epithelial cells. 



THE BLADDER. 



The bladder, the reservoir in which the urine is received from the renal 

 ducts and retained until discharged through the urethra, is essentially a 

 muscular sac, lined with mucous membrane and partly covered with perito- 

 neum, a layer of connective tissue loosely uniting the mucous and muscular 

 coats. From within outwards, four coats are distinguishable the mucous, 

 the submucous, the miiscular and \hzfibrous. 



The mucous coat closely resembles that of the renal duct, consisting 

 of a fibro-elastic tunica propria covered by transitional epithelium. The 

 details of the latter are materially affected by the degree of contraction or 

 distention to which the mucosa is subjected, since it is probable that the 

 impression of a many-layered epithelium is based on the examination of sec- 

 tions of the strongly contracted organ. As ordinarily seen, the deepest cells 

 are irregularly columnar, the ones of the middle layers polyhedral or club- 

 shaped, and the surface cells somewhat flattened, with their deeper aspect 

 modelled by the subjacent elements, over and between which fit depressions 

 and projections. Although definite glands can hardly be said to exist, in 

 the vicinity of the vesical trigone and of the urethral orifice the tunica 

 propria contains small epithelial pockets or crypts, from the bottom of which 

 short branched tubules, lined with low columnar cells, extend into the sur- 

 rounding stroma. These rudimentary glands have been interpreted as 

 representing abortive prostatic tubules, which have become displaced during 

 the development of the lower segment of the bladder from the uro-genital sinus. 



The submucous coat, loose and elastic, permits free gliding of the 

 mucous membrane over the muscular tunic when readjustment becomes 

 necessary during contraction and the mucosa is strongly wrinkled. It is 

 composed of bundles of fibrous tissue, interwoven with numerous elastic 



