50 INTRODUCTION. 



along the v*liole length of the spinal column, is 

 largest in the middle portion, points at both ends, 

 and exhibits not the slightest cerebral development 

 at the anterior extremity. A shallow groove runs 

 along the mesial line of the upper part of the 

 cord, which is partially filled with a black pigment. 

 The cord may be traced with great ease to within 

 one-sixteenth of an inch of the anterior extremity 

 of the column, and far from dilating into a brain, 

 it becomes, on the contrary, extremely slender. 

 When the spinal cord is examined under a high 

 magnifying power, it is found to be composed en- 

 tirely of nucleated cells, very loosely attached to 

 each other, and enclosed in an exceedingly delicate 

 covering of pia mater. From fifty-five to sixty 

 nerves pass off from each side of the cord, having 

 no double roots, but inserted at once into its 

 edges, in the form of simple cords. These nerves 

 divide into two sets of branches, which are seve- 

 rally distributed upon the dorsal and ventral aspects 

 of the body. The first pair is excessively minute, 

 and is distributed about the superior angles of the 

 mouth ; and the second pair corresponds to the tri- 

 facial of more highly developed animals. The pe- 

 culiarities of the structure of the spinal cord are not 

 less remarkable than those of its configuration; nor 

 is it an easy matter to understand how a spinal 

 cord destitute of primitive fibres or tubes, and alto- 

 gether composed of isolated cells, can transmit 

 influences of any kind in any given direction. 

 Thirdly, that an animal should be correctly ar- 



