390 THE SENSORIAL FUNCTIONS. 



trunk (n,) which is afterwards divided and subdivided in 

 the course of its farther distribution, both to the muscular 

 and the sentient organs of the body. Each of these spinal 

 nerves also sends branches to the ganglia of the sympathetic 

 nerve, which, as was formerly described, passes down on 

 each side, parallel and near to the spine. 



Enlargements of the spinal marrow are observed 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 uni- 

 form diameter. In Fishes, these enlargements are in pro- 

 portion to the relative size and muscularity of the lateral fins, 

 and correspond to them in their situation. The Piper Gur- 

 nard ( Tri'^la 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 enlargements, 

 presenting a double row of tubercles. Fishes which possess 

 electrical organs have a considerable dilatation of the spinal 

 marrow, answering to the large nerves w^hich are distributed 

 to those organs. Birds which fly but imperfectly, as the 

 Gallinaceous tribe and the Scansoi^es, 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 considera- 

 ble than the posterior in birds which have great power of 

 flight. In the Dove, of which the brain and whole extent of 

 the spinal marrow are shown in Fig. 449, the enlargements 

 (w and l) corresponding to the wings and legs respectively, 

 are nearly of equal size. In Quadrupeds, we likewise find 

 the relative size of these enlargements corresponding to that 

 of fore and hind extremities. When the latter are absent, 

 as in the Cetacea, the posterior dilatation does not exist. 



The brain (b) may be regarded as an expansion of the an- 

 terior or upper end of the spinal marrow; and its magnitude, 



