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 Ammoccetes, instead of two Mauthnerian fibres, a number of 

 giant-fibres are found. They are called Mullerian 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 Mullerian 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 Eetzius, 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 Eetzius, two specially large 



