1909.] 



Cerehral Cortex of Echidna, etc. 



123 



section was then substituted for tiie micrometer and all the fibres lying 

 within the outline of the squares were marked. Thus the number of fibres 

 in a definite area of the section was ascertained. This was done two or three 

 times in different parts of the nerve. By dividing the total area of the nerve 

 by the area for which the number of nerve fibres is known and multiplying 

 by the latter number, an estimate of the total number of fibres in the nerve 

 may be obtained, the accuracy of which will vary with the ratio of the known 

 to the unknown, and with the degree of uniformity with which the fibres are 

 scattered throughout the nerve. 



The following is a concrete example, namely the optic nerve in Series A : — 

 Under the low power (16 mm. appochr. obj., 8 comp. occ), its area was 

 found to be 0"295 sq. mm. The number of nerve fibres for O'Ol sq. mm. was 

 found by the high power (2 mm. appochr. imm. obj., 4 comp. occ.) to be in 

 three different places 974, 942, 992, respectively, which have an average 969. 



969x0-295h-0-01 = 28,585. 



The results obtained by these two methods are summarised in the table. 



Table showing Number of Nerve Fibres in the Cranial Nerves of two 

 Specimens of Echidna. 



Nerve. 



Brain A. 



Brain B. 



II 



28,585* 



46,566* 



Ill 



1,978 



1,849 



IV 



4U4 



223 



V 



-t 



43,410* 



VI 



671 



474 



VII 



4,281 



3,535 



VIII 



24,565* 



23,523* 



IX 



2,740 



2,170 



X 



3,617 



3,042 



XI 



3,227 



2,180 



XII 



5,214 



2,865 



A, Echidna from Zoological Gardens. B, Echidna sefosa from Tasmania. 

 * Number estimated by the method described in text. 



f Owing to there liaving been only a very short piece of the Vth nerve in Specimen A available 

 for sectioni&ing, which was about as long as it was broad, it was accidentally cut longitudinally 

 and not transversely, and was thus useless for the purpose of counting the fibres. 



