GENERAL SUEVEY OF THE VEETEBEATA. 619 



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 con- 

 <j!ected with the muscular 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 YEKTEBEATA a distinct system of 

 nervous centres lodged among the viscera, appropriated to the perform- 

 ance of the automatic functions, and presiding over those involuntary 

 movements of the body upon which depend the operations 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, destined to supply the organs belonging 

 to digestion, circulation, and secretion, thus forming extensive ramifi- 

 cations, formerly distinguished by the name of the sympathetic nerve, 

 but now more properly considered as a distinct system presiding over 

 organic life, as the former is connected with the phenomena of animal 

 life. 



(1658.) With the increased development 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 be- 

 comes slowly adapted to appreciate 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 vertebrate 

 division of the animal world, which must not be omitted in this pre- 

 paratory survey of their organization. Their organs of digestion and 

 nutrition are constructed according to a different type, and upon a more 

 enlarged plan than in any of the classes enumerated in the preceding 

 chapters ; and parts are superadded to the digestive apparatus which in 

 lower tribes had no existence. In addition to the usual subsidiary glands, 

 namely, the salivary and the hepatic, a third secretion is poured into 

 the intestine along with the bile, derived from the pancreas, a viscus 

 which we have not as yet met with. Throughout all the MOLLTJSCA we 

 have found the bile secreted by the liver to be separated from arterial 

 blood, as are the other secretions of the body ; but in the YERTEBRATA 



