762 
MR. A. SANDERS ON THE ANATOMY OF THE 
aqueduct of Sylvius. The commissure in question is derived from two separate regions. 
The anterior part connects the two sides of the anterior end of the floor of the 
ventricle of the optic lobe ( Thalamus opticus , Stieda). The posterior part connects 
the anterior termination of the central longitudinal columns of the cord, or at least 
of some few fibres which are the continuation forward of those columns. The deeper 
commissure (figs. 3 and 24, c.pr.) is derived from the region about the anterior part 
of the floor of the optic lobe ; it then passes downwards and backwards, and passing 
through the “ nucleus rotundus ” of the hypoarium, turns downwards and forwards, and 
crosses over in front of the infundibulum to the corresponding point on the opposite 
side. 
Ccjvier (28), Gottsche (34), and oilier anatomists considered that the former 
homologised the anterior commissure of the cerebrum, but it appears to me that its 
position corresponds better with the posterior commissure ; its situation at the 
posterior end of the third ventricle, and behind the infundibulum, points in that 
direction ; and if, as Stieda maintains, the region round this ventricle comprising 
the anterior part of the floor of the optic ventricle homologises the thalami optici, 
this interpretation would be strengthened, for the commissure in question connects the 
two sides of that region. 
Longitudinal Columns and their Anterior Prolongations. 
The anterior columns of the cord are composed of two bundles of large fibres (fig. 1, 
&c.), one of which is situated close to the ventral edge, the other is situated internally 
beneath the substantia gelatinosa centralis ; they are separated from each other by 
the ventral commissures ; the former may be called the ventral longitudinal columns, 
and the latter the central longitudinal columns. In the latter are situated the giant 
fibres, two in number, one in each column. These were first described by Mauthner 
(50) ; they are oval in section, measuring 0'016 millim. by 0 - 012 millim. ; each is 
contained in a separate tube of connective tissue, which is much larger and more 
regular in outline than the tubes surrounding the remaining fibres of these bundles ; 
the other nerve-fibres in these columns vary in size from O'OIO millim. by 0‘005 
millim. to 0‘003 millim. in diameter: thus there is a gradation in size from the small 
nerves to those nearly as large as the Mauthner fibres. The smaller ones are more 
numerous in the lateral part of the ventral columns. The Mauthner fibres do not 
seem to correspond, as Mauthner thought, to the large fibres in the spinal cord of the 
myxinoid fishes, which are described by Muller (36) as fiat band-like fibres, which 
these are not. 
The Mauthner fibres (fig. 8) decussate at about opposite the posterior end of the 
origin of the trifacial, after which they disappear entirely, and I could trace them no 
farther ; probably they break up into fine fibrillae of which they are most likely composed. 
In support of this opinion I may cite a case in which I saw one of these fibres in the 
