CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
753 
sometimes it presents a tendency to the oval form ; these nuclei often show a distinct 
nucleolus. About ten or twelve of these cells occupy the thickness of this sixth 
layer; towards the anterior part of the tectum this layer passes continuously into 
the fornix. (I shall adopt this term in preference to “ torus longitudinalis ” used by 
Fritsch, without attaching any homological meaning to it, but simply as indicating 
the form of the part to which it is applied ; it has been in use since Carus first 
discovered the part, and therefore has the sanction of antiquity in its favour.) The 
other species (fig. 18, d.) of nervous elements in the tectum is found in the second 
layer ; these are long, fusiform, cell-like swellings of the radial fibres, which are finer 
at the inner end than at the outer ; it is somewhat difficult to measure their length 
owing to the gradual transition between the fibre and the cells, it may however be 
said to vary between 0*018 millim. and 0*040 millim. ; the width is more constant, 
being seldom more than 0*005 millim., but occasionally reaching as much as 
0*007 millim. ; the nucleus measured about 0*005 millim. by 0*003 millim. These 
cells are not enclosed in a space, but the neuroglia is in apposition to their external 
surface. The fibre which passes from the inner end of these cells is the finer, and can 
be traced into the smoothly striated third layer ; the fibres from the sixth layer can 
be traced into the same stratum on the other side ; from which circumstance the pre- 
sumption arises that the small cells of the internal layer stand in connexion with the 
fusiform bodies just described. Stieda (58) places these cells in his third or striated 
layer, and considers that they belong to the neuroglia “ grunclsubstanz.” This does not 
appear to be quite correct ; they are situated, in fact, in the layer which he terms “ die 
aussere Langsfaserschicht.” Their nervous character can scarcely be doubtful ; they 
resemble in fundamental structure the cell-like swellings on the fibres of the olfactory 
lobe, and also the Purkinje cells of the second layer of the cerebellum, as the latter 
would appear if drawn out and stretched so as to be made long and thin. The 
outer process of these cells can be traced under favourable circumstances into the 
outer finely-granular layer of the tectum. 
The modifications undergone by these layers in various parts of the tectum are 
as follows : — 
At the outer side of the entrance of the crura lobi optici the fifth layer disappears 
as a distinct stratum, being only formed by the fibres of that part running in an 
inward direction towards the central commissure of the tectum. Anteriorly towards 
the inner end the first layer disappears. On the outer side all the layers, except the 
first, third, and sixth, are obliterated by the fibres of the optic tract. Further back, 
at about the middle of the tectum, longitudinal fibres appear which are intercalated 
between the first and second layers ; others occur between the fourth and fifth layers 
towards the inner edge ; these latter gradually die out and extend only about one-third 
of the width of the tectum. 
The cells of the sixth layer are continuous internally with those of the fornix 
at the anterior termination of the tectum. The fornix consists of two longitudinal 
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