MYELON OF FISHES. 271 



in the vertebrate subkingdom. In all other Fishes the fore part 

 of the neural axis receives the vagal, trigeminal, and special sense 

 nerves, and developes and supports ganglionic masses, principally- 

 disposed in a linear series parallel with the axis : this part is the 

 ' brain ' (encephalon) ; the rest of the axis retaining its columnar 

 or chord-like character is the ' myelon,' and being lodged in the 

 canal of the spinal column, it is usually defined as the medulla 

 spinalis (spinal marrow, or spinal chord). 



In the Lamprey the myelon is flattened, opaline, ductile, and 

 elastic, as in the Lancelet and other Dermopteri : in typical 

 Fishes it is inelastic and opacpie, cylindrical or sub-depressed ; 

 of nearly uniform diameter, gradually tapering in the caudal 

 region to a point in heterocercal Fishes, but swelling into a small 

 terminal o-anolion l in most homocercal Fishes. 



The Hunteriau preparation of the skate (Raia Batis) 2 shows 

 a slight (brachial or pectoral) enlargement of the myelon, Avhere 

 the numerous large nerves are sent off to the great pectoral fins : 

 a feebler brachial enlargement may be noticed in the Sharks. 

 I have not recognised it in Osseous Fishes, not even in those with 

 enormous pectorals adapted for flight, e. g. Exoccetus and Dac- 

 tylopterus : in the latter the small ganglionic risings upon the 

 dorsal columns of the cervical region of the myelon receive nerves 

 of sensation from the free soft rays of the pectorals, and the 

 homologous ganglions are more marked in other Gurnards 

 (Triglce), which have from three to five and sometimes six pairs 3 , 

 e. g. in Trigla Adriatica. Similar myelonal cervical ganglions 

 are present, also, in Polynemi/s. In the heterocercal Sturgeon 

 there is a feeble expansion of the myelon at the beginning of the 

 caudal region, whence it is continued, gradually diminishing to a 

 point along the neural canal in the upper lobe of the tail. In 

 some bony fishes (Trout, Blenny) the caudal ganglion is not quite 

 terminal, and is less marked than in the Cod or Bream, in which 

 it is of a hard texture, but receives the last pair of spinal nerves. 

 The absence of this gano-lion in the Shark shows that it relates 

 not to the strength of the tail but to its form, as depending on 

 the concentration and coalescence of the terminal vertebra? ; 

 except, indeed, where such metamorphosis is extreme, as, e. g. in 

 Orthar/oriscus mola, and where it affects the entire condition of 

 the myelon, which has shrunk into a short, conical, and, according 



1 liii. p. 6; liv. p. 2G (in the Cod). 



2 xx. vol. iii. p. 40, prep. No. 1347. 



3 lv. pi. 2, fig. 4, p. 106; and liii. p. G, pi. 2, fig. 24, 25. 



