THE NERVOUS SYSTEM OF THE BIRD 185 
as to the exact origin and termination of certain fibres which appear to correspond to those 
indicated by this observer. 
III. Posterior Commissure and Lamina Commissuralis Mesencephali 
(a) Posterior Commissure. — This is seen as a large bundle cut transversely in sagittal sections 
(Figs. 15 and 18, Plate VI). In Fig. 17 the fibres are seen curving backwards to join the 
posterior longitudinal bundle. In horizontal sections (Fig. 7, Plate IV) it is seen in the roof of 
the mesencephalon, and in transverse sections (Fig. 28, Plate VII) the deeper portion can be seen 
curving inwards around the aqueduct to the region of the descending fibres of the posterior 
longitudinal bundle. 
(b) Lamina Commissuralis Mesencephali. — A wide commissure limited in front by the 
posterior commissure, and behind by the decussation of the fourth nerves. 
It is seen in sagittal sections (Figs. 15-18, Plate VI) cut transversely, and its great width 
is obvious in horizontal sections (Fig. 7, Plate IV). It is a lamina of white fibres extending 
between the two optic lobes, the fibres reaching far into the roof of the ventricle of the vesicle 
(Fig. 28, Plate VII). In this commissure lie the large ganglion cells of the roof of the aqueduct, 
which were originally described by Stieda.* Stained by Held's modification of the Nissl 
method, we have found that they are grouped as parallel rows of large oval cells as large as those 
forming the nucleus of the Ilird nerve. Their nucleus is large, and their protoplasm contains 
numerous Nissl bodies (Fig. 55, Plate X) ; it is believed that they give origin to the radix 
mesencephali of the trigeminus. BRANDist describes in considerable detail certain fibres in the 
region of the medulla oblongata turning round so as to surround the Vth nucleus, and giving off 
branches which pass into that nucleus, and states that this tract can be traced upwards into the 
region of the roof of the middle brain, and in some of our Marchi preparations, after lesion of the 
optic lobe, certain degenerated fibres can be seen passing from the region of these cells downwards 
into the medulla. 
IV. Transverse Fibres of the Optic Vesicle 
(a) The commissure of the optic vesicle described as above. 
(b) Deeper transverse fibres from the interior of the optic vesicle which arise much anterior 
to the previous group, and pass across to the wall of the aqueduct (Figs. 9 and 10, Plate V). 
(c) Tractus tecto-spinalis. — Fibres arising in a similar position to the preceding, and passing 
more obliquely than the transverse fibres ; they form part of the descending mesencephalic fibres, 
extending downwards to the cord (Fig. 11, Plate V). 
(d) The diagonal tract. — A well-marked bundle more ventrally situate to any of the 
preceding, and passing in a diagonal region to the region of the oculomotor nucleus (Figs. 12 and 
13, Plate V). 
We have not found any description of fibres corresponding to these, which may represent 
tracts connecting the sensory with the motor side of the visual apparatus. 
Stieda, loc. cit. 
■f- Brandis, 'Arch, fiir Mikr. Anat.,' 30, 43, 1894. 
Y 
