190 



ESSENTIALS OF ZOOLOGY 



to the exterior of the body are the MalpigMan corpuscles^ each made 

 up of a glomerulus and a Bowman's capsule, and the coiled urinifer- 

 ous tubules which discharge the excretion through collecting tubules 

 into the ureter at the pelvis of the kidney. This canal leads to the 

 cloaca in most vertebrates below mammals (excepting some fish), or 

 to a urinary bladder in the mammals. 



The wall of each Bowman's capsule is very thin and readily per- 

 mits diffusion of water and dissolved materials from the blood into 

 the cavity of the uriniferous tubule on the opposite side of the mem- 

 brane. The glomerulus carries arterial blood from the afferent ar- 

 terial branch and discharges it into the efferent arterial branch. The 

 latter soon spreads into a capillary network which surrounds the 

 convoluted portions of the uriniferous tubule. Water constitutes 



Fig. 57. — Diagram of Malpighian corpuscle and renal secretion. G, Malpighian 

 corpuscle with two layers of Bowman's capsule enclosing the space i; af, vas 

 afferens vessels supplying the glomerular capillaries which fill the capsule to line 

 j, to which the inner wall of the capsule is closely applied ; ef, vas efferens which 

 drains the glomerular capillaries and supplies the capillaries ; ca, surrounding the 

 convoluted tubule, C; a, blood pressure in glomerular capillaries; c, the urine filtrate 

 pressure in the tubule ; r, the reabsorption from the uriniferous tubule into the 

 capillaries, ca ; h, the urine on its way to the colleCLin^ tubule: b, the restraining 

 osmotic pressure' of the protein molecule. (From Zoelhout, Textbook of Physiology, 

 The C. V. Mosby Company.) 



the largest volume of materials to be excreted in most animals, ex- 

 cept in some desert forms where water is conserved and the ex- 

 cretion is in crystalline form. Water is eliminated by lungs, skin, 

 alimentary canal, and kidneys. In man the quantity of sweat dis- 

 charged may amount to two or three liters a day. In the dog, 

 which has no sweat glands, the water eliminated by the lungs, 

 through panting, is proportionately greater than in man. The kid- 

 neys are the most important organs in the excretion of water, and 

 the amount they eliminate is inversely proportional to the amount 



