758 
MR. A. SANDERS ON THE ANATOMY OF THE 
decussation. I could discover no fibres that remained on the same side, neither are 
there any which traverse the chiasma from one nerve to the other without passing on 
into the brain ; neither does the commissura transversa Halleri, which is stated by 
Gottsche* * * § to be present behind the chiasma in Teleostei, appear to be present here. 
With regard to the deep origin of the nerve (figs. 18 and 22), the principal part is 
derived from the anterior end of the optic lobe, where the fibrils of origin occupy 
nearly two-thirds of the width, comprising in fact the whole of the first or external 
layer. The fibrils appear to be derived from the cells mentioned in the description of 
the lobe in question. In addition to this origin some of the fibres are derived from 
the hypoarium. BELLONCit denies this origin, and is of opinion that bundles belonging 
to the anterior and posterior commissura transversa have been credited to the chiasma 
by mistake; but, unfortunately for this idea of Bellonci, these structures do not 
exist in specimens of Plagiostomata that I have examined. My reason for including 
the hypoarium as part of the origin of the optic nerve is this ; if very thin horizontal 
sections of the chiasma be examined at the point where its posterior margin abuts on 
the hypoarium, several fibres are to be seen passing into it, dispersing through that 
body, and not returning into the chiasma. If they were simply passing through to 
some other part they would retain their formation as bundles and would not separate 
into single fibrillse. I have not actually traced any of them into a cell, but, neverthe¬ 
less, as it is, I think this arrangement shows that they end, or begin rather, in the 
hypoarium. 
Third pair (fig. 15).—The oculomotorii nerves are derived from a small group of 
nerve cells situated on each side of the fissure on the floor of the ventricle of the 
optic lobe; from these ganglia a well-defined bundle passes downward and outward, 
and emerges from the inferior surface, one on each side of the raphe. AhlbornJ 
however describes a decussation, or, as he prefers to call it, a chiasma of the roots 
of this nerve in the Petromyzou. It seems hardly probable that a decussation should 
be present in this animal and not in the Plagiostomata, but of course it is just 
possible that he is right; if so, the Petromyzon is differently organized to the other 
members of the class Pisces. It is true that there is a decussation of fibres in 
the neighbourhood of this nerve, which might have deceived Ahlborn, but it is a 
distinct formation, and has nothing to do with the oculomotorii; it extends for some 
distance both before and behind them, and appears to homologize with the commissura 
ansulata § in the Teleostei. In the region between the roots of this nerve there 
are apparently two sections of this commissure : a dorsal part the fibres of which 
decussate, and a ventral part the fibres of which simply pass over from one side to 
* Op. cit ., p. 442, fig. 8. 
f “ Ueber den Ursprung des Nervus Opticus, &c.” ‘ Zeitschr. f. Wissensch. Zool.,’ vol. 35, 1880, p. 24. 
X “ Untersuchung fiber das Gebirn der Petromyzonten.” 4 Zeitscbr. f. Wissensch. Zool.,’ vol. 39, 1883, 
p. 271. 
§ Gottsche, op. cit., p. 439. 
