38 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



of capillaries, called a glomerulus, around which the beginning of the tubule 

 forms a very thin cup-shaped inclosing membrane, the Bowman's capsule. 

 Through the opening of the cup an artery enters the glomerulus, and a vein 

 leaves it; opposite this point the uriniferous tubule begins. You will probably 

 have to examine a number of Malpighian bodies in order to see all of these points. 

 The tubule after leaving the Malpighian body winds in a complicated way 

 through the substance of the kidney and finally empties into a collecting tubule 

 which opens into the ureter. 



F. STRUCTURE OF THE SPINAL CORD 



Examine slide "Frog spinal cord" (see Holmes, p. 286). The spinal cord, 

 as we have seen, consists of the central gray matter, composed of nerve-cell 

 bodies, and the peripheral white matter, composed of nerve-cell processes. In 

 the center of the gray matter is a small canal, the central canal. The four corners 

 of the gray matter are produced into processes, known as the dorsal and ventral 

 horns, or cornua of the gray matter. The ventral horns are considerably broader 

 than the dorsal horns, and contain the large motor cells which have already 

 been examined. The spinal cord is nearly separated into two symmetrical 

 halves by the dorsal and ventral fissures, which extend from the middle of the 

 dorsal and ventral sides in toward the central canal. The dorsal fissure is shallow 

 and is continued toward the central canal by a narrow septum, composed of 

 loose tissue. The ventral fissure is much deeper and extends nearly to the 

 central canal; it incloses an artery, the ventral spinal artery. 



The white matter consists mostly of cross-sections of the nerve fibers, which 

 take their origin from the cells of the gray matter. In addition to the nerve 

 fibers, the white matter contains scattered cells, which are mainly the cells of 

 the neuroglia, a peculiar kind of connective tissue, found only in the nervous 

 system. The neuroglia forms a network supporting the nervous structures. 



The cord is surrounded by a sheath, the pia mater, composed of connective 

 tissue, and containing numerous blood vessels. 



Draw the section of the cord, showing the above-mentioned features. 



The instructor will be glad to lend sections of other organs of the frog to 

 students who care to see them. 



