748 
MR. A. SANDERS ON THE ANATOMY OF THE 
in P.etromyzon ; the ganglion he looks upon as eorresponcling to the ganglion 
habenulas, and the bundle of fibres to Meynert’s fibres. This interpretation may 
possibly be correct; the relation of the parts is not antagonistic to, but rather favours 
this view, but the ganglion interpedunculare is not to be found in any specimens 
examined. 
Lobi Optici. 
The ventricle of the optic lobe in ScyIlium (figs. 15 and 18) is partially divided into 
two by a longitudinal depression of the roof, corresponding internally to the external 
longitudinal furrow, which gives to the outer contour the appearance of a double lobe 
(the corpus bigeminum), an appearance which is not verified by the internal construction, 
the venti’icle being single ; in this respect the optic lobe of the Scyllium resembles 
the tectum lobi optici of the Teleostei. There are, however, considerable differences 
between the two, more especially in the macroscopic aspect. The torus longitudinalis 
is absent, the transverse commissure which crossed its base alone remaining ; the 
thickness of this lobe exceeds that of the tectum actually and proportionately. On 
the other hand, in compensation, the structure is simplified. 
The tori semicirculares, which are well developed in the bony Fishes, are wanting in 
the Plagiostomata, so that in the latter the floor of the ventricle is slightly concave ; 
the furrow in the central line is common to both. 
There are three layers to be made out more or less clearly in these lobes. The first 
or external layer contains numerous cells embedded in a granular neuroglia; they 
attain their maximum in this layer, but extend also into the next in diminishing 
numbers. Toward the outer edge a few scattered cells only are to be seen, but they 
gradually increase in number, and in the centre they attain their maximum. These 
cells are mostly spherical in shape, and measure on an average about 8p in diameter- 
in addition to these, there are found in this layer, in much smaller numbers, fusiform 
cells placed with their long axes in a radial direction. These cells correspond in 
every respect with the fusiform cells which are found in the tectum lobi optici of the 
Teleostei. Like all the bodies here distinguished by the term cell, they have a 
nucleus and a nucleolus ; the nucleus is not coloured so deeply as the remainder of the 
cell contents by the staining fluid. This shows their correspondence in structure with 
the cells of the cerebrum, hypoarium, and the granular and molecular layers of the 
cerebellum, and differentiates them from the motor cells of the spinal cord and the 
medulla oblongata and other ganglion cells, in which the nncleus is stained of a deeper 
tint than the remainder of the cell contents. They are generally broader, but not so 
long as those of the Teleostei; one end gives off a comparatively thick fibre going 
toward the inner surface of the lobe, the other end, being pointed, probably gives off a 
fine process, judging from the analogy with the cells in Teleostei, but if so it escaped 
notice from being broken off’in the seel ions. The position of the nucleus varies, some¬ 
times being near the pointed, at others near the broad end of the cell. 
