THE PHYSIOLOGY OF THE URINARY ORGANS. 243 



of the urine is the most compound, holding in suspension, 

 in addition to its other chemical constituents, a remark- 

 able one, termed urea. Urine in colour is yellow, of a 

 lighter or deeper tint ; at times it is transparent ; at 

 others turbid in the extreme ; its odour is peculiar, some- 

 times offensive : and its specific gravity is somewhat greater 

 than that of water. The phenomena attending the forma- 

 tion and ejection of urine are numerous and important: 

 one of the most interesting of these is the rapidity of its 

 formation after liquid aliments have been received; and 

 the very early detection in the urine of matters taken into 

 the stomach. The regularity of the urinary secretion is a 

 notable phenomenon ; it is always going on, as well when 

 sleeping as when waking. The complete distention of the 

 bladder occasions an instinctive attempt at evacuation ; but 

 the act remains partly under the influence of the will, and 

 is much regulated by habit. The expulsion of the urine 

 from the bladder is thus accounted for — 



The neck of the bladder is composed of yellow elastic 

 tissue. To this the long order of muscular fibres are attached. 

 Distention excites the muscular fibres, which are of the white 

 or organic kind, into action; the bladder having urine 

 within it, more or less, according to its irritability, when 

 the muscular fibres contract. The obUque diminish the 

 diameter of the organ; while the longitudinal muscle, 

 attached to the neck, pulls up the fundus near to the open- 

 ing ; and at the same time drags the elastic tissue, which is 

 the natural cork to the organ and would of itself remain 

 firmly closed, violently apart. The urine thus released, and 

 forced out by the energetic contraction of its muscles, 

 enters the urethra, and thence is discharged upon the 

 ground. 



The oblique entrance of the ureters into the bladder, 

 these ducts passing for some distance between the layers of 

 muscular fibre before they enter into the organ, acts in both 

 directions as a perfect valve. When the bladder is relaxed 

 there can be no pressure upon its sides, therefore the urine 

 enters freely, and has no disposition to return ; but so soon 

 as the bladder is full, or becomes distended, pressure is 

 made upon its sides, and thus the further entrance of urine 

 is prevented ; while the perfect closure of the orifice effec- 



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