FINAL REMARKS . 489 
Their absence in those fishes, in which a well-developed tail-fin is 
also absent, increases the probability of the truth of Fritsch’s original 
conclusion that these giant-fibres are associated axis-cylinders for 
certain definite co-ordinated movements of the fish, especially for the 
lateral movement of the tail. 
In Ammoceetes, instead of two Mauthnerian fibres, a number of 
giant-fibres are found. They are called Miillerian fibres, and arise 
from giant-cells which are divisible into two groups. The first group 
consists of three pairs situated headwards of the level of exit of the 
trigeminal nerves. Two of these lie in front of the level of exit of 
the oculomotor nerves, and one pair is situated at the same level as 
the origin of the oculomotor nerves. The second group consists of 
a number of cells on each side at the level of the entrance of the 
fibres of the auditory nerves. 
The Miillerian fibres largely decussate, as described by Ahlborn, 
and then become the most anterior portion of the white matter of the 
spinal cord, forming a group of about eight fibres on each side 
(Fig. 73). A few fibres are also found laterally, and slightly 
dorsally, to the grey matter. These giant-fibres pass down the spinal 
cord right to the anal region ; their ultimate destination is unknown. 
Mayer considers that in the first part of their course they correspond 
to those tracts of fibres known as the “ posterior longitudinal bundles” 
in other vertebrates. I imagine, therefore, that the spinal part of their 
course represents the two antero-lateral descending tracts. The 
second group of giant-cells, which appears to have some connection 
with the auditory nerves, may represent “Deiter’s nucleus.” The 
whole system is probably the central nervous part of a co-ordination 
mechanism, which arises entirely in the pro-otic or prosomatic region 
of the brain—the great co-ordinating and equilibrating region par 
excellence. 
If we turn now to the arthropod it is a striking coincidence that 
in the crayfish and in the lobster the work of Retzius, of Celesia, 
of Allen, and of many others demonstrates the existence of an 
equilibration-mechanism for the swimming movements of the tail- 
muscles, which is carried out by means of giant-fibres. These giant- 
fibres are the axis-cylinder processes of giant-cells, situated exclusively 
in the brain-region, and they run through the whole ventral ganglionic 
chain in order to supply the muscles of the tail. In the ventral 
nerve-cord of the crayfish, according to Retzius, two specially large 
