266 FUNCTIONS OF THE NERVOUS SYSTE3I. 



Invertebrata. In the latter it has seemed but a mere appendage to the rest of 

 the organism, a mechanism superadded for the purpose of bringing its various 

 parts into more advantageous relation. On the other hand, in the Vertebrata 

 the whole structure appears subservient to it, and designed but to carry its 

 purposes into operation. Again, in the Invertebrata, we do not find any 

 special adaptation of the organs of support, for the protection of the Nervous 

 System. It is either inclosed, with the other soft'parts of the body, in one 

 general hard tegumentary envelope, as in the Echinodermata and Articulata; 

 or it receives a still more imperfect protection, as in the Mollusca. In the 

 latter, the naked species are destitute of any means of passive resistance, and 

 the Nervous System shares the general exposed condition of the whole body ; 

 and it is not a little remarkable that, in the testaceous kinds, the portion of 

 the body containing the most important nervous centres should be protruded 

 beyond the shell, whilst the principal viscera are retained within it. Now, 

 in the Vertebrata, we find a special and complex bony apparatus, adapted in 

 the most perfect manner for the protection of the Nervous System ; and it is, 

 in fact, the possession of a jointed spinal column, and of its cranial expansion, 

 which best characterizes the group. 



340. When we look more particularly at the Nervous Centres themselves, 

 we perceive that they combine the general characters of those of the Articulata 

 with those of the Mollusca ; the locomotive powers of the former (compara- 

 tively reduced, however, in activity) being united with the complex nutritive 

 system of the latter ; and we find this combination manifested, not only in 

 the organs themselves, but in the Nervous System, which stands in so close 

 a relation with them. The Spinal Cord of Vertebrata is evidently the ana- 

 logue of the ventral columns of Articulata. It is a continuous ganglion, con- 

 taining two portions as distinct as the two tracts in the Articulata; a fibrous 

 structure, which is continuous between the Brain and the spinal nerves, and 

 thus resembles the white tract in Insects ; and a ganglionic portion, princi- 

 pally composed of gray matter. Into this gray matter, as in the ventral gan- 

 glia of Insects, a part of the roots of the spinal nerves may be traced; whilst 

 others seem to pass on continuously to the brain. At the upper extremity of 

 the Spinal cord (commonly termed the Medulla Oblongata) we find the 

 ganglia and nerves of special sensation ; and the organs which these supply 

 are placed in immediate proximity with the entrance to the alimentary canal, 

 holding the same relation to it as in the Mollusca. But in addition to these 

 we find two ganglionic masses in all Vertebrata, to which we have no distinct 

 analogue in the lower classes the Cerebral Hemispheres, and the Cerebel- 

 lum. With the development of the former of these, as already remarked, 

 the perfection of the reasoning powers appears to hold a close relation ; that 

 of the latter seems connected with the necessity which exists, for the adjust- 

 ment and combination of the locomotive powers, when the variety of move- 

 ments performed by the animal is great, and the harmony required among 

 them is more perfect. Upon these points, however, we shall hereafter 

 dwell. 



341. The Visceral system of nerves now assumes a more distinct form. 

 It docs not share the protection of the Spinal column; hut its ganglia lie for 

 the most part in the general cavity of the trunk. These ganglia, which 

 are doubtless the independent centres of some of the nerve-fibres proceed- 

 ing from them, arc much more numerous than is commonly supposed. It 

 appears from recent researches, that we are to regard as belonging to the 

 Visceral or Sympathetic system, not only the Semilunar and Cardiac ganglia 

 (which seem to be its principal centres), with the chain of cranial, cervical, 

 thoracic, lumbar, and sacral ganglia, which are in nearer proximity to the 



