2 C. JUDSON HERRICK 



NECTURUS 



The external form and the more important internal structures 

 of the brain of Necturus have been well described in Kingsbury's 

 excellent paper published in 1895. More detailed figures of the 

 surface anatomy and the relations of some of the cranial nerves 

 were published by McKibben in 1913. 



It is difficult to fix the brain of Necturus for either macroscopic 

 or microscopic examination without some distortion of its form, 

 even when the brain is left in situ in the cranium during fixation. 

 McKibben ('13, p. 155) has given instructions for reducing this 

 distortion to a- minimum. Figure 1 presents a dorsal view of 

 a dissection of the cerebellar region and figures 2 and 3 a view from 

 behind and very slightly from above of another specimen. Both 

 of these brains were fixed in situ in Zenker's fluid in which the ace- 

 tic acid was replaced by 10 per cent formalin before the dissec- 

 tions were completed and the chorioid plexuses removed. Figures 

 12 to 16 were drawn from cross sections of a specimen in which the 

 distortion is very slight save for some dorso-ventral flattening. 

 Figures 4 to 11 were drawn to illustrate the fiber tracts and the 

 rather complicated technical procedure involved in their prepara- 

 tion has caused more distortion of the external form. 



The recessus lateralis rhombencephali of Necturus is a wide 

 expansion of the rostral end of the fourth ventricle, a part of which 

 extends forward on each side of the midbrain as a blind anterior 

 diverticulum. The 'auricular lobe' thus constituted is in a general 

 way similar to that of selachian and some ganoid fishes. Figure 

 2 is drawn from a point of view which reveals the full extent of 

 the lateral recesses and anterior diverticula. The walls of the 

 anterior diverticulum are massive on all sides (figs. 4, 5, 6, 12, 13, 

 17). Part of its medial wall is fused with the lateral wall of the 

 mesencephalon, but its anterior end is free (figs. 21, 22). The 

 roof of each lateral recess (except the anterioV diverticulum) is 

 membranous and plexiform, this portion of the chorioid plexus of 

 the fourth ventricle being evaginated and extending both lateral- 

 ward and forward far beyond the limits of the underlying massive 

 wall (figs. 14, 21), as Kingsbury has mentioned. That portion of 



