VERTEBRATA. 485 



sition as they were considered to be by the older anatomists : each 

 of them has, in fact, been found to arise from the spinal cord by 

 two distinct roots, one derived from the anterior, the other from 

 the posterior column of the corresponding side ; so that each nerve 

 is evidently made up of two distinct sets of filaments, one set 

 communicating with the motor, the other with the sensitive tracts ; 

 and thus every nerve derived from the spinal cord is a compound 

 structure, being composed of filaments distinct in office, although 

 enclosed in the same sheath, some being connected with the mus- 

 cular movements, the others with sensation. But in addition to 

 the cerebro-spinal ganglia and the symmetrically arranged nerves 

 emanating therefrom, that are distributed to the organs of sensa- 

 tion and movement, there exists in the Vertebrata a distinct system 

 of nervous centres lodged among the viscera, appropriated to the 

 performance of the automatic functions, and presiding over those 

 involuntary movements of the body upon which depend the ope- 

 rations connected with nutrition. These ganglia are variously 

 distributed, being situated in the head, the neck, the thorax, and 

 the abdomen ; and from them arise large plexuses of nerves, des- 

 tined to supply the organs belonging to digestion, circulation, and 

 secretion ; thus forming extensive ramifications, formerly distin- 

 guished by the name of the sympathetic nerve, but now more pro- 

 perly considered as a distinct system presiding over organic life, 

 as the former is connected with the phenomena of animal life. 



With the increased developement of the nervous system in the 

 vertebrate classes we find the organs of the senses assume a pro- 

 portionate perfection of structure and regularity of arrangement. 

 The auditory apparatus, of which we have seen only rudiments in 

 the lower animals, gradually becomes more and more elaborately 

 organized : the eyes, now invariably two in number* are lodged in 

 cavities formed for their reception by the osseous framework of the 

 face ; and exhibit, in the simplicity of their structure, a higher type 

 of organization than any we have hitherto examined. Organs of 

 smell, also double, but of very variable construction, are likewise 

 constantly present. The tongue becomes slowly adapted to ap- 

 preciate and discriminate savours; and the sense of touch, the most 

 generally diffused of all, is especially conferred upon organs of 

 different kinds peculiarly adapted to exercise this faculty. Thus 

 with increased intelligence higher capabilities of enjoyment are 

 allotted, and sagacity developes itself in proportion as the nervous 

 centres expand. But there are minor points, characteristic of the 



