NERVOUS SYSTEM. VEKTEBKATA. 487 



SPINAL CORD. 



The spinal cord is a direct backward continuation 

 medulla oblongata. It extends for a variable distance tou 

 the tail, being in lower forms coextensive with the vertebral 

 column, but in the higher showing a more or less marked 

 shortening in relation to it. In the latter case, a degenerate 

 vestige of the hinder end of the cord is still present in the pos- 

 terior parts of the neural canal, surrounded by a bundle of spinal 

 nerve-roots (cauda equina). Extreme shortening of the cord 

 occurs in isolated cases in widely separated groups (Diodon, 

 Erinaceus) . 



In transverse section, the cord is usually oval or circular, 

 rarely flattened and ribbon-like (Cydostomi) or quadrangular 

 (Rays). The central canal is lined by an ependyma and 

 surrounded by a layer of grey matter. 



In many fishes there is no definite boundary between the ;_ 

 and white matter, but in most cases it is clearly defined and has 

 in transverse section the form of a St. Andrew's cross. 



In the lower groups, where the sense-organs of the skin are 

 mainly innervated by certain cranial nerves, the dorsal (sensory) 

 horns of grey matter are relatively very weak. Ganglion-cell* 

 occur chiefly in the ventral (motor) horns, evenly distributed 

 throughout their length. The presence of limbs, if of sufficient 

 size and activity to require nerves markedly larger than those 

 of the trunk, is accompanied by a swelling of the cord due to an 

 increase in the number of nerve-elements at the point of origin of 

 the limb-nerves. In a similar way, any other increased m-rvc- 

 supply occasions a corresponding local thickening of the cord : 

 (<?. #., Gurnard, tactile pectoral fin-rays. Gynuiotus, electric 

 organ). Conversely, diminished nerve-supply, as in the thoracic 

 region in the Chelonia, is accompanied by diminution in the size 

 o the cord. 



The cord is more or less extensively united by tracts to the 

 different regions of the brain. In lowly forms this connection 

 is comparatively slight, being restricted mainly to the ventral 

 tracts (fasciculi longitudinales posteriores) and fillet. Hence 

 in these the cord is far more independent of the brain in its 



