50 INTRODUCTION. 



along the wliole length of the spinal column, is 

 largest in tne 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, vt'hich 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 

 €dges, 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- 



