V THE URINARY APPARATUS 177 



ureter is lined by an epithelium consisting of several layers 

 of cells. Outside of these is a muscular coat made up of 

 unstriated muscle-fibres, arranged in two layers and 

 surrounded externally by some fibrous connective tissue. 

 In front of the ureters is a single aperture which leads 

 into the canal called the Urethra (Fig. 50, 2), by which 

 the cavity of the bladder is placed in communication with 

 the exterior of the body. The openings of the ureters 

 enter the walls of the bladder obliquely, so that it is much 

 more easy for the fluid to pass from the ureters into the 

 bladder than for it to get the other wa}', from the bladder 

 into the ureters. 



Mechanically speaking, there is little obstacle to the 

 free flow of fluid from the ureters into the bladder, and 

 from the bladder into the urethra, and so outwards ; but 

 certain muscular fibres arranged circularly around the part 

 called the "neck" of the bladder, where it joins the 

 urethra, constitute Avhat is termed a sphincter, and are 

 usually, during life, in a state of contraction, so as to close 

 the exit of the bladder, while the other muscular fibres 

 of the organ are relaxed. 



It is only at intervals that this state of matters is 

 reversed ; and the walls of the bladder contracting, while 

 its sphincter relaxes, its contents, the urine, are dis- 

 charged. But, though the expulsion of the secretion of 

 the kidneys from the body is thus intermittent, the 

 excretion itself is constant. The urinary fluid is pro- 

 pelled drop by drop along the ureters by rhythmic 

 contractions which pass along their walls in the direction 

 of the bladder where it accumulates, until its quantity is 

 sufficient to give rise to the uneasy sensations which 

 compel its expulsion. 



3. The Structure of a Kidney. — When a longitudinal 

 section of a kidney is made (Fig. 51), the upper end of the 

 ureter {U) seems to widen out into a basin-like cavity (P), 

 which is called the pelvis of the kidney. Into this 

 sundry conical elevations, called the pyramids (Pi/) 



N 



