SO THE FROG 



In the ventral cornua or horns of gray matter (portions 

 of the gray matter situated ventro-laterally) may be seen a 

 few cell-bodies of nerve cells of unusually large size; proc- 

 esses from these cell-bodies go to form fibers of the ventral 

 or motor roots of the spinal nerves. The fibers of the dorsal 

 or sensory root have their origin in the ganglion of this root, 

 previously described. 



In the white matter the longitudinal nerve fibers are, of 

 course, cut transversely. Examine the cut ends of these 

 fibers under high power; the axis cylinder of each fiber ap- 

 pears as a minute dark spot surrounded by a clear space oc- 

 cupied by the medullary substance. 



The nervous elements of the cord are bound together by 

 stellate neuroglia cells and by processes which arise from the 

 tapering outer ends of the ependyma cells; these processes 

 branch repeatedly and some of them extend to the periphery 

 of the cord. Occasional capillaries may appear in the sec- 

 tions. 



The nerve cord is closely surrounded by the following 

 coverings : (a) an outer firm membrane, comprising the 

 inner layer of the dura mater; and (&) an inner vascular 

 membrane, corresponding to both the arachnoid and the 

 pia mater of human anatomy. Both membranes are some- 

 times torn away in making the preparation. 



Under low power, draw a cross-section of the cord. The 

 drawing should be about 12 centimeters in diameter. Under 

 high power, make a special drawing of the cell body of a 

 nerve cell. 



Be sure you understand the paths into and through the 

 cord of an impulse involved in a reflex act (see below). 



C. Reflex Action.- -This experiment will be performed 



