674 Journal of Agriculture. [10 Nov., 1908. 



the capsule is composed is such that water and the salts of blood, and pre- 

 sumably also urea, can pass through, but the other constituents, with the 

 corpuscles, are held back and remain in the blood stream. Such a filter 

 as this is not unknown in physical science. A clay cell soaked in hot 

 gelatine solution and then allowed to cool, so that the gelatine se^s in the 

 pores, can be used as a filter, allowing water and the simple constituents 

 present in blood tO' pass through, but retaining the corpuscles and albumens 

 and other complex constituents. In the capsule, therefore, there collects a 

 watery filtrate from the blood : this escapes from the opening mentioned 

 above and travels down a tortuous tube and finally, being collected in 

 special ducts, is poured out as urine into the pelvis of the kidney. But 

 a great change occurs in the fluid as it passes through the tortuous or 

 "convoluted" tube. The blood which has been concentrated and which 

 has lost some of its dissolved constituents in the glomerulus leaves the 

 latter by a small vessel and then flows in a network of capillaries which 

 embraces the convoluted tube. Here a good part of the water and some 

 of the salts are re-absorbed back into the blood ; but typical urinary con- 

 stituents are left alone. The lining cells of the tortuous tube also have 

 the power of picking out from the blood certain urinary ingredients such 

 as the pigments or foreign substances which have not passed through the 

 glomerular membrane, and of adding these to the urine within. Even 

 haemoglobin which has escaped from corpuscles, when these are diseased 

 or in any way broken up, is treated as a foreign body and is thrown out 

 into the urinary stream. The cells also of Ihe tortuous tube have also the 

 faculty of standardizing the blood as to reaction and salt content. The 

 blood finally leaves the meshwork of the capillaries purified and standard- 

 ized whilst the fluid in the tortuous tube is now ordinary urine and is 

 ejected as such. The kidney contains manv thousand glomeruli with 

 their attendant tortuous tubes and plexus svstems. 



It will be evident from the above statements that the flow of urine will 

 be increased — 



1. If the blood pressure be raised ; 



2. If the kidney vessels are dilated admitting more blood to the 

 glomeruli and at higher pressure ; this occurs with drugs like turpentine 

 and caffeine ; 



3. If absorption in the tortuous tubes is hindered; this occurs after 

 administration of sulphates; 



4. If the blood be watery as occurs after copious drinking unaccom- 

 panied by heavy sweating. 



The converse of these will reduce the flow of urine. 



The urine collected in the pelvis of the kidney flows along a tube called 

 the URETER being driven by peristaltic contractions. Each ureter enters 

 the bladder so obliquely that a valve is formed effectually preventing any 

 back-flow. The bladder is a hollow organ well supplied with elastic 

 tissue and smooth muscle in its walls. Its nerve supplv and its capacity 

 relative to the animal's size, vary with different species and are connected 

 with the animal's habits. Thus the cow can void urine while walking, the 

 horse as a rule only when stationary, whilst the carnivore, being a hunting 

 animal, is fitted with an unusually capacious bladder and one more under 

 voluntary control, so that it can stalk its prev without betraving its pre- 

 sence by passing urine. The expulsion of urine from the bladder through 

 the urethra is a reflex action excited bv tension of the bladder wall or 



