178 EXCEETION. 



the blood sent to the kidneys is for the purpose of furnishing 

 water and the excrementitious principles of the urine, and 

 but little is used for ordinary nutrition. Secretion appears 

 to have no marked influence upon the consumption of oxygen 

 and the production of carbonic acid. 



Physiological Anatomy of the Urinary Passages. The 

 chief physiological interest attached to the anatomy of the 

 urinary passages is connected with the discharge of the urine 

 from the kidneys into the bladder, and the process of mictu- 

 rition ; and it will be necessary, consequently, to give but a 

 brief account of the structure of these parts. 



The excretory ducts of the kidneys, the ureters, commence 

 each by a funnel-shaped sac, the pelvis, which is applied to 

 the kidney at the hilum. This sac presents little tubular 

 processes, called calices, into which the apices of the pyra- 

 mids are received. The ureters themselves are membranous 

 tubes of about the diameter of a goose-quill, becoming much 

 reduced in calibre as they penetrate the coats of the bladder. 

 They are from sixteen to eighteen inches in length, passing 

 from the kidneys to the bladder behind the peritoneum. 

 They have three distinct coats ; an external coat, composed 

 of fibrous tissue, the ordinary white fibres mixed with elas- 

 tic fibres of the small variety ; a middle coat, composed of 

 different layers of non-striated muscular fibres ; and a mucous 

 coat. 



The external coat requires no special description. It is 

 continued into the calices and is continuous with the fibrous 

 coat of the kidney at the apices of the pyramids. 



The fibres of the muscular coat present two principal 

 layers ; an external longitudinal layer, and an internal 

 transverse, or circular layer, to which is added near the 

 bladder a layer of longitudinal fibres, internal to the circu- 

 lar fibres. 



The mucous lining is thin, smooth, and without any fol- 

 licular glands. It is thrown into slight longitudinal folds, 



