( 378 ) 



CHAPTER VIII. 



COMPARATIVE PHYSIOLOGY OF THE NERVOUS SYSTEM. 



1. Nervous Systems of Invertebrated Jlnimals. 



OUR knowledge of the exact uses and functions of the 

 various parts which compose the nervous system, and espe- 

 cially of its central masses, is unfortunately too scanty to 

 enable us to discern the correspondence, which undoubtedly 

 exists, between the variations in the functions and the di- 

 versities in the organization. The rapid review which I 

 propose to take of the different plans, according to which 

 the nervous system is constructed in the several classes of 

 animals, will show that these central masses are multiplied 

 and developed in proportion as the faculties of the animal 

 embrace a wider range of objects, and are carried to higher 

 degrees of excellence. 



In none of the lowest tribes of Zoophytes, such as Sponges, 

 Polypi, and Medusse, have any traces of organs, bearing 

 the least analogy to a nervous system, been discovered; not 

 even in the largest specimens of the last named tribe, some 

 of which are nearly two feet in diameter. All these ani- 

 mals give but very obscure indications of sensibility; for 

 the contractions they exhibit, when stimulated, appear to 

 be rather the effect of a vital property of irritability than 

 the result of any sensorial faculty. Analogy, however, 

 would lead us to the belief that many of their actions are 

 really prompted by sensations and volitions, though in a de- 

 gree very inferior to those of animals higher in the scale of 

 being: but whatever may be their extent, it is probable that 

 the sensorial operations in these animals take place without 



