THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA 



399 



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 

 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 few 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 

 excreted by the skin. Most of the water to be excreted is taken 

 from the blood in Malpighiaji corpuscles. 



Some of the nitrogenous wastes are excreted in the form of am- 

 monium salts and some free or combined amino acids. However, 

 most of the ammonia which results from protein metabolism is con- 

 verted into urea in the liver and is carried in that form to the kid- 

 neys where it is removed from the set of capillaries ramifying over 

 the convoluted tubules by a process of true secretion. According 

 to this idea, the urine which consists of urea, various salts, other 

 soluble materials, and water is excreted by different parts of the 

 uriniferous tubule. The substances which are excreted by the kidney 

 are not formed there, but are merely removed from the blood by 

 this organ. 



The Nervous System. — The nervous system in this type of animal 

 is composed of a hrain and spinal cord forming the central nervous 

 system; nerves extending to all parts of the body, ganglia which are 

 groups of nerve cell bodies outside the central nervous system, and 

 the sense organs which serve for receiving stimuli are usually 

 grouped together under the name peripheral nervous system. A por- 

 tion of this latter division, consisting of two longitudinal trunks 

 with ganglia distributed along them, lies parallel to the spinal cord, 

 and constitutes the sympathetic system. Each ganglion has a connec- 

 tion with the adjacent spinal nerve or cranial nerve as the case 



