PHYSIOLOGICAL ANATOMY OF THE URINARY PASSAGES. 407 



the vein, and the quantity of oxygen diminished, with a corresponding increase in the 

 proportion of carbonic acid. These observations show that during secretion most of the 

 blood sent to the kidneys is for the purpose of furnishing water and the excrementitious 

 principles of the urine, and that but little is used for ordinary nutrition. Secretion ap- 

 pears 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 with the process of micturition; 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 tu- 

 bular processes, called calices, into which the apices of the pyramids are received. The 

 ureters themselves are membranous tubes of about the diameter of a goose-quill, becom- 

 ing much reduced in caliber 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 tis- 

 sue, the ordinary white fibres mixed with elastic 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 prolonged 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 longitudi- 

 nal layer, and an internal transverse, or circular layer, to which is added near the blad- 

 der a layer of longitudinal fibres, internal to the circular fibres. 



The mucous lining is thin, smooth, and without any follicular glands. It is thrown 

 into slight longitudinal folds, when the tube is flaccid, which are easily effaced by dis- 

 tention. The epithelium exists in several layers and is remarkable for the irregular 

 shape of the cells. They present, usually, numerous dark granulations and one or two 

 clear nuclei with distinct nucleoli. Some of the cells are flattened, some are rounded, 

 and some are caudate, with one or two prolongations. 



Passing to the base of the bladder, the ureters become constricted, penetrate the coats 

 of this organ obliquely, their course in its walls being a little less than an inch in 

 length. This valvular opening allows the free passage of the urine from the ureters, but 

 compression or distention of the bladder closes the orifices and renders a return of 

 the fluid impossible. 



The bladder, which serves as a reservoir for the urine, varies in its relations to the 

 pelvic and abdominal organs as it is empty or more or less distended. When perfectly 

 empty, it lies deeply in the pelvic cavity and is then a small sac, of an irregularly trian- 

 gular form. As it becomes filled, it assumes a globular or ovoid form, rises up in the 

 pelvic cavity, and, when excessively distended, it may project into the abdomen. When 

 the urine is voided at normal intervals, the bladder, when filled, contains about a pint 

 of liquid ; but, under pathological conditions, it may become distended so as to con- 

 tain ten or twelve pints, and, in some instances of obstruction, it has been found to con- 

 tain even more. The bladder is usually more capacious in the female than in the male. 

 It is held in place by certain ligaments and folds of the peritoneum, which it is unneces- 

 sary to describe in this connection, but which are so arranged as to allow of the various 

 changes in volume and position which the organ is liable to assume under different de- 

 grees of distention. 



The anatomy of the coats of the bladder possesses a certain amount of physiological in- 

 terest. These are three in number. The external coat is simply a reflection of the peri- 

 toneum, covering the posterior portion completely, from the openings of the ureters to the 

 summit, about one-third of the lateral portion, and a small part of the anterior portion. 



