1884.] NATURAL SCIENCES OF PHILADELPHIA. 26T 



and superior (processus ad cerebrum) peduncles of the higher 

 vertebrate brain. 



The scarcit}-, if not absence, of nerve cells in the Amphiuma 

 or Menopoma cerebellum, renders it difficult to understand the 

 meaning of tliese peduncles, unless we regard the cerebellum here 

 as in large part a decussational system, composed of fibres 

 crossing from one side of the brain to the other. It may be 

 added that the frog's cerebellum is richly cellular. 



The Optic Nerves (fig. 8). — No fibres have as yet been followed 

 from the optic lobe (Mesencephalon) to enter the optic tracts, 

 although there can be little doubt that they are present ; but the 

 fibres in the thalami arise in a manner which points, almost with 

 certainty, to the important fact that in the Amphibia the decus- 

 sation of the optic tracts is incomplete. In other words, part of 

 the fibres of each optic nerve enter from the chiasma, i. e., from 

 the opposite side of the brain, part enter from the same side of the 

 brain. (1.) The fibres supplj'ing the chiasma, arise from cell 

 masses in the upper lateral portions of the thalami, and sweep 

 around the sides of the thalami, partly encircling the main longi- 

 tudinal fibre system (crura cerebri) ; they pass downwards and 

 obliquely forwards, enter the chiasma, and apparently pass to the 

 nerve of the opposite side. (2.) In the floor and lower lateral cell 

 masses of the thalami arise smaller bundles of fibres, which pass 

 beneath the longitudinal system, above and then in front of the 

 chiasma to enter the optic nerve of the same side. They can be 

 traced by following successive sections forwards, but do not 

 interdigitate with the fibres of the chiasma, as in the figure which 

 combines the results of a series of sections. If this fact is con- 

 firmed by other observers, it will show that the partial decussation 

 of the optic tracts is an early, if not a primitive condition, instead 

 of being peculiar to the higher mammals, as has been generally 

 maintained. 



The Commissures. 



TJie Prsecommissura (fig. 9). — In the frog's brain ^ it has been 

 found that there are two divisions of this commissure : a posterior, 

 connecting the lower portions of the hemispheres, and an anterior, 

 connecting the upper median walls. Both have been found in 

 Menopoma^ the latter arching upwards at the sides, and, as is 

 clear in fig. 4, it forms on either side the posterior boundary'' of 



1 Stieda, Iqc, qU., p, 308, 



