554 THE SENSORIAL FUNCTIONS. 



thetic nerve, which, as was formerly described, 

 passes down on each side, parallel and near 

 to the spine. 



Enlargements of the spinal marrow are ob- 

 served in those parts (w and l, Fig. 449), which 

 supply the nerves of the extremities ; the increase 

 of diameter being proportional to the size of the 

 limbs requiring these nerves. In Serpents, 

 which are wholly destitute of limbs, the spinal 

 marrow is not enlarged in any part, but is a 

 cylindrical column of uniform diameter. In 

 Fishes, these enlargements appear to have a re- 

 lation to the size of the organs of motion or sen- 

 sation, and correspond to them in their situation. 

 Thus in the Trigla lyra (the Red or Piper Gur- 

 nard), and the Trigla gurnardus (the Grey Gur- 

 nard), there are, at the commencement of the 

 spinal marrow, numerous enlargements, present- 

 ing a double row of tubercles, (as seen in the 

 space between m and s, Fig. 451). The nerves 

 from these tubercles supply the detached rays, 

 or feelers, anterior to the pectoral fin. Fishes 

 which possess electrical organs have a consider- 

 able dilatation of the spinal marrow, answering 

 to the large nerves which are distributed to those 

 organs. Birds which fly but imperfectly, as the 

 Gallinaceous tribe and the Scansores, have the 

 posterior enlargement much greater than the an- 

 terior ; a disproportion which is particularly re- 

 markable in the Ostrich. On the contrary, the 

 anterior enlargement is much more considerable 



