Hardesty, spinal Nerves of the Frog. 65 



I. Summary. 



1. A gross examination of the different spinal nerves 

 shows marked variations in their architecture (Plate VI). 



2. The number of fibers in the ventral roots decreases 

 from the spinal cord towards the spinal ganglion. 



3. The number of fibers in the dorsal roots decreases 

 from the spinal ganglion towards the spinal cord. 



4. The section of the nerve trunk immediately distal to 

 the spinal ganglion (dorsal branches excluded) contains a great- 

 er number of fibers than are fouud in a section of the trunk 

 further distal. 



5. The decrease in the number occurs among the smaller 

 fibers of the nerve. 



6. The general explanation of these relations is found in 

 the fact that the fibers arising from the spinal ganglion grow, on 

 the one hand, towards the spinal cord by way of the dorsal 

 root and, on the other, towards the periphery by way of the 

 nerve trunk ; and, that the fibers of the ventral root grow from 

 the spinal cord towards the periphery. 



7. In frogs of increasing weight, the fibers of the dorsal 

 root increase in number more rapidly than do those of the ven- 

 tral root. 



8. The sum of the fibers in the trunk and dorsal branches 

 combined, exceeds in every case and by a considerable amount 

 the sum of the fibers in the two roots. 



9. The excess of fibers in the trunk and dorsal branches 

 seems to be correlated both with the absolute and proportional 

 size of the dorsal branches. 



10. The method herein employed for the enumeration of 

 nerve fibers possesses several features which commend it as 

 more trustworthy than any of those previously employed. 



II. Introduction. 



The number, size and arrangement of the medullated nerve 

 fibers constituting the peripheral nervous system in vertebrates 

 presents many problems worthy of investigation. 



The problem which forms the basis of this paper is a de- 



