CENTRALIZATION OF NERVOUS APPARATUS. 



action of light on different vegetable colours serves to prove that there is scarcely a ray 

 which is not implicated in these processes ; sometimes it is the red, sometimes the yel- 

 low, sometimes the hlue. So, too, with rays differing specifically, the rays of heat of 

 different refrangibilities, and the tithonic rays. 



397. In the lower orders of animal life, the controlling influence of the same agents 

 which are thus so active in the vegetable world is well marked; the movements of the 

 polygastric infusorials are well known to be directed, to a certain extent, by light. 

 There seems to be a diffused sensibility to that agent possessed by the entire surface 

 of these beings. Comparative anatomists have traced how, from this, which is the ob- 

 scurest development of specific nervous action, the nervous material begins to be col- 

 lected in centres, and locomotive or respiratory ganglia make their appearance. In the 

 case of insects, the metamorphoses of which are open to our inspection, as they pass 

 successively through the larva and pupa states, and finally reach their imago, or per- 

 fect condition, so far as the nervous system is concerned, all the transitions tend to a 

 concentration ; the development of a new instinct, or the production of increased loco- 

 motive power, is at once expressed by changes in the magnitude or position of the ner- 

 vous ganglions, or their connecting cords. It is true, that as soon as this concentra- 

 tion appears, it brings with it new qualities, and, in the more elevated orders of life, one 

 of the great functions of the nervous system seems to be the establishing of sympathetic 

 connexions between organs which are carrying on processes that are essentially differ- 

 ent ; between the digestive, the respiratory, the secreting apparatus. Among plants, 

 this high state of centralized organization is not required ; the sap, driven, as we have 

 seen, by mere mechanical forces, makes its way from the spongioles to the leaves, and 

 then commences its descent. This circulatory movement is the only bond of union 

 between the various parts of a vegetable system. For this reason, therefore, botanists 

 are fully justified in their assertion, that a tree is not a simple individual, but rather a 

 colony of individuals. It possesses no interior nervous system, the office of which is 

 to bring into connexion parts that are distant, because the type upon which it is con- 

 structed requires no such machinery. 



398. But there are considerations which enable us to trace analogies between the 

 mode of action of the nervous principle, even in the most elaborate forms, with those, 

 more obscure, which belong to the vegetable world. As has been fully proved by phys- 

 iologists, all the nerves of special sensation take their origin amid the minute ramifica- 

 tions of bloodvessels ; and it is the changes which occur in the circulating fluid which 

 impress a specific action on the nervous terminations an action at once transmitted to 

 the centres, and there disposed of. Thus, by the heart, venous blood, from which the 

 elements of carbonic acid have to be expelled, is thrown into the minute ramification 

 of the pulmonary artery, and there, coming in contact with the terminal fibres of the 

 par vagum, it impresses upon them an influence an influence which is in an instant 

 propagated to the respiratory nervous centres, and there is expended in producing a 

 reflex effect. Through the proper nervous channels, the diaphragm, the intercostals, 

 and the various muscles engaged in respiration, are put in action ; the capacity of the 

 chest increases ; a few cubic inches of atmospheric air enter the trachea, and fill the 



