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 are in proportion to 

 the relative size and muscularity of the lateral 

 fins, and con'espond to them in their situation. 

 The Piper Gurnard {T. Igla lyra), which is a 

 species of flying fish, having very large pectoral 

 fins, that portion of the spinal marrow supplying 

 their muscles with nerves (as seen in the space 

 between m and s, Fig. 451), has numerous en- 

 largements, presenting a double row of tubercles. 

 Fishes which possess electrical organs have a 

 considerable dilatation of the spinal marrow, 

 answering to the large nerves which are dis- 

 tributed to those organs. Birds which fly but 

 imperfectly, as the Gallinaceous tribe and the 

 Scansores, have the posterior enlargement much 

 greater than the anterior ; a disproportion which 

 is particularly remarkable in the Ostrich. On 

 the contrary, the anterior enlargement is much 

 more considerable than the posterior in birds 

 which have great power of flight. In the Dove, 



