48 
G. H. Parker 
lion with the brain terminate in each of tbese places in fibrillations 
and are connected with ganglionic cells only in the region of the 
fourth optic ganglion. 
This Statement, however, may not be without its exceptions, for, 
according to Retzius (90, pag. 37), in some cases at least, the optic 
fibres are connected with cells in the brain. Granting that there 
are such fibres, then terminations in the fourth ganglion without 
ganglionic cells (Flg. 53) would be a normal relation, for it is im- 
probable that a nerve fibre is connected with ganglionic cells both 
in the brain and in the fourth optic ganglion. Although I am un- 
able to give a final decision on this question, I feel confident that, 
w^hile a few optic nerve fibres may possess ganglionic cells located 
in the brain, the great majority of them are connected with cells in 
the fourth optic ganglion only. Whether these fibres have their cells 
in the one position or in the other, they represent the last system of 
neurons between the retina and the brain, neurons of the fifth order. 
Before leaving the subject of the optic nerve, it is necessary to 
say a word or so about certain of its peculiarities. Although most 
of its fibres are true optic fibres, it contains a few belonging to other 
categories. Immediately before the fourth ganglion is reached, the 
optic nerve gives off a few fibres to the muscles that move the optic 
stalk. These fibres can be traced easily to their distribution among 
the muscle fibres, and I have no hesitation in pronouncing them 
motor fibres. There are probably not more than eight or ten of 
them in the whole nerve. They are unconnected with ganglionic 
cells outsjde the brain, and, as they are probably united with cells 
within this structure, it may be possible that these were the fibres 
that Retzius observed to be connected with cerebral cells. 
Another peculiarity of the optic nerve is the possession of a few 
delicate fibres that, taking a superficial course, extend over the sur- 
face of the optic ganglia even to the first of these and terminate in 
cells from v^hich a few branching processes extend still further 
over the ganglionic mass (PI. 2 Fig. 58). These nervous elements, 
which represent single neurons, terminate in the brain with the 
optic fibres and yet apparently have no direct connection with the 
optic mechanism, for they do not pass through the decussations nor 
reach the retina. I regard them as similar to nervi nervorum, i. e., 
deep-seated sensory neurons surrounding the delicate optic ganglia. 
Judging from the number that appear stained, these elements cannot 
be very numeroup, possibly as numerous as the motor fibres. 
