470 VITAL MOTION. 



animals consist of substance as soft as mucous or gelatinous tissue ; 

 for instance, the polypi, most radiaria, some entozoa and the 

 infusoria: yet the former will swim or crawl, attach or detach 

 themselves, and seize prey; the infusoria swim rapidly, turn, and 

 avoid each other, and possess distinct muscles. Sedatives and 

 stimulants affect these movements of vegetables and of such 

 animals like those of large animals. Such vegetables and animals, 

 as well as minute insects and infusoria, which evidently perform 

 what in large animals we should term muscular movements, show 

 that living structure, though so soft that it cannot be regarded 

 as precisely similar to the flesh of large animals, to muscular 

 fibre, to what is termed muscle, is capable of living contrac- 

 tion. Such minute voluntary actions are attended, Raspail de- 

 clares, in one infusory animalcule the rotifer, by thickening 

 during contraction of the muscular cylinders running from its 

 head to its tail, and by tenuity of them when they lengthen. In 

 animals possessing muscles, many parts, not apparently muscular, 

 contract, and instantaneously and forcibly, by a living force. Such 

 are minute vessels and canals of all kinds. These lose their con- 

 tractile power, like muscles, immediately or soon after death. 

 Some structures are most adapted for contraction, as muscles ; 

 others not at all, as tendons and bones : but others, though not 

 evidently muscular, possess the faculty in various degrees ; and 

 to expect distinct muscular fibre in every excitable part would 

 be erroneous. 



The vital power of motion, whether sensible, as in the heart 

 and voluntary muscles and the leaves and flowers of many vege- 

 tables ; or insensible, (except by its effect on contained fluids) as 

 in the minute vessels of vegetables and animals, may have the 

 term excitability restricted to it (see supra, p. 25.), and thus will 

 be distinguished from sensibility, to which the idea of motion is not 

 necessary, as seen in the terminations of the optic and olfactory 

 nerves, though motion may follow sensation ; and sensation again is 

 not necessary to motion, for not only do many animal motions oc- 

 cur without sensation, but vegetables are utterly destitute of sens- 

 ation. The term irritability was peculiarly given by Haller d to the 



c 1. c. 497. Dr. Tiederaann says that, with a microscope and strong lens, 

 he observed contractions and expansions in the simplest infusoria, 1. c. dlxxv. 

 though previously he had asserted that neither could be detected in them, 

 cccclxiii. 



d " See Haller on the irritable parts of the human body, Commcntar, Soc. Sc. 

 Gotting. t. ii. 



