140 ESSENTIALS OF ZOOLOGY 



liver, pancreas, kidneys, and spleen; thoracicolumhar ganglia with 

 fibers to muscles of hairs, skin arterioles, eye, viscera, blood vessels, 

 and certain urogenital structures; sacral ganglia with fibers to ex- 

 ternal genital organs, urinary bladder and large intestine. Auto- 

 nomic impulses either stimulate or inhibit action in the organs sup- 

 plied. Each autonomic ganglion is connected to the nerve of its 

 level by a band of fibers called the ramus communicans. 



Excretory System 



The principal organs constituting this system are a pair of kidneys, 

 a pair of ureters, a single bladder and a single urethra. These 

 organs relieve the blood of urea, various salts, excess water, etc., 

 and discharge these substances from the body as urine. 



The kidneys are bean-shaped structures located dorsal to the peri- 

 toneum in the lumbar region of the trunk. The two kidneys are not 

 exactly opposite each other on the two sides of the midline, the left 

 one being about a half a length more posterior. At the medial side 

 of each kidney is a depression called the liilum where the renal 

 blood vessels and ureter join it. The ureters are tubules leading, 

 one from each kidney, to the urinary bladder. The internal struc- 

 ture of the kidney consists of a more peripheral or outer layer, the 

 cortex, which lies just internal to the capsule, and the inner medullary 

 portion. The cortex contains the Malpighian corpuscles which are 

 essential structures in receiving urine from the blood. The medul- 

 lary portion is composed largely of tubules. These tubules converge 

 in the papillae which lead to the sinus in the hilum. Here the 

 papillae empty into the widened end of the ureter, which fills the 

 renal sinus and is called the renal pelvis. The Malpighian (renal) 

 corpuscle is composed of the inverted, swollen end of a uriniferous 

 tubule and a mass of blood capillaries enclosed by it. The former 

 is known as Bowman's capsule and the knot of capillaries enfolded 

 by it is the glomei^ulus. 



The urinary bladder is a thin-walled oblong sac in the posterior 

 portion of the abdominal cavity. It receives the urine from the 

 ureters. The narrowed neck which continues from it to the exterior 

 is named the urethra. The urethra becomes tubular, and in the 

 male extends through the penis to its tip, serving both excretory 

 and reproductive functions through most of its length. In the 



