947 



with the addition that the imi)regiiatioii in a thirty per oeiit. AgNO, 

 solution was made specially long — over ten days and nights. The 

 impregnated pieces were imbedded in parafiin and were cut into 

 conveniently thick sections, 5—15 (i. A nnmber of spinal ganglia 

 were cut in uid)roken series of sections, 10—15 n in thickness. 

 Cross sections were placed on the suh-dnral parts of the spinal 

 nerves, partly close to (centrally of) the spinal ganglia and partly 

 close to the spinal cord. In the preparation, in which the si)inal cord 

 and the spinal ganglia had been lixed in sitn in the canalis med. 

 spinalis, cross-sections were cut right from the caudal end as far 

 in the direction of the cranium as the sub-dural part of the nerve 

 roots had a caudal course ^ this was, as a rule, np to the posterior 

 third and the posterior half of the thoracal vertebral column. The 

 rest was cut into sagittal sections, during which the microscope was 

 used to verify that conveniently situated i)arts of the segmental 

 nerves were 'present in the sections. These sections were made 

 5_10 ft thick. In determining the nnmber of nerve tibres in the 

 cross sections I used a Leitz microscope (tripod G.H.) with a 

 cross-table, an oil-immersion Vi, a, and an ocular IV (Leitz) 

 with the enclosed squared glass plates. It appeared to be necessary 

 to work with such a great magnifying power in determining the 

 number of nerve fibres in order to be able to disintegrate those 

 parts of the preparation m which the nerve fibres were most 

 close, especially in the young animals. Before beginning to count, 

 the square-ocular and the cross-table were adjusted so as to prevent 

 as far as possible unexpected displacements and miscalculations 

 arising from these. Repeated calculations with the same preparations 

 have also shown that the errors in calculation that we are concerned 

 with are small - always less than ten per cent, as a rule not more 

 than five per cent. At first I attempted an approximate method in 

 deciding the number of nerve fibres in the cross-sectioned nerve 

 roots. 1 counted each nerve fibre in a few hundred squares and 

 found the average number. I then counted the number of squares 

 in a cross-section and multiplied this by the average number. This 

 method appeared, however, to give values that were too uncertain, 

 because nerve libres of different thicknesses were very unevenly 

 distributed. In order, therefore, to obtain sufficiently exact values, 

 I was thus compelled to count every nerve fibre in the whole cross- 

 section — a method that was certainly tronblesome, but necessary 

 in this case, especially with young animals. In counting I always 

 began at the top and at the left, both in the preparation and in 

 the field of the squares, taking care that all the nerve fibres which 



