494 Animal Biology 



globelike reiial (Malpighian) corpuscles, each of which is composed of 

 (1) a coiled mass of thin-walled capillaries arising from the reiial arteries, 

 each mass being called a glomerulus, and (2) a thin double-walled en- 

 closing glomerular capsule ( Boivm,an s capsule) which is the beginning 

 of a renal tubule (Fig. 245). The convoluted renal tubules travel irregu- 

 larly and empty into the straighter collecting tubes which in turn pass 

 the urine into the basinlike pelvis of the kidney from which it goes out 

 through the ureter (Fig. 245). The blood from the glomeruli eventually 

 passes from the kidney through the renal veins into the ascending vena 

 cava. When examined microscopically, the medulla consists of cone- 

 shaped renal pyramids whose apices are known as renal papillae. The 

 collecting tubes empty at the apices of the papillae, which vary from eight 

 to eighteen in number. 



The glomeruli extract the wastes from the blood, thus helping" to main- 

 tain the normal composition of the blood. The kidneys selectively ex- 

 tract almost all the protein waste, most of the salts not required by the 

 blood, and about half of the excess water. They also extract foreign sub- 

 stances such as toxins. The quantity of urine secreted in twenty-four 

 hours varies, but the normal average for a healthy adult is 1,200 to 1,500 

 c.c. 



The contractions of the muscles in the walls of the ureters cause the 

 urine to pass toward the muscular bladder located in the pelvic cavity. 

 The bladder normally holds about one pint, and the contraction of its 

 three layers of muscles forces the urine to the exterior through the tubu- 

 lar urethra. 



VIII. COORDINATION IN MAN AND 

 SENSORY EQUIPMENT 



All living protoplasm is necessarily irritable or subject to stimulation. 

 A stimulus is any external or internal substance, material, or condition 

 which affects a cell or group of cells, thereby setting up a change known 

 as a response. General types of stimuli are chemical, electrical, thermal, 

 mechanical, radiant, and osmotic. General types of responses are move- 

 ment, secretion, thermal, chemical, electrical, and photic. The respon- 

 sive mechanisms of man are complex and varied. The three steps in- 

 volved are as follows: (1) a special structure called a receptor must be 

 stimulated; (2) some method of conduction of the effects of stimulation 

 to (3) a specialized structure called an effector which must respond in 

 some way. Receptors in man are frequently specialized epithelial cells 



