VITAL PROPERTIES OF THE TISSUES. 89 



simple excitability, or the property of receiving impressions from, or 

 being excited by, certain stimuli; whilst, in the latter, it assumes 

 more special forms of reception and reaction, constituting true sensi- 

 bility, to which must be added volitional and excito-motory or reflex 

 power. Moreover, both forms of tissue are able not merely to receive, 

 but also to transmit, the effects of impressions, whilst the nerve-cells 

 especially originate internal actions. That nerve-fibres possess, in a 

 marked degree, the power of conducting the effects of impressions, 

 either inwards to proper receptive or reflective nervous centres, en- 

 dowed with special excitabilities, which are then called into play, or 

 outwards to the muscular organs, which being endowed, as we have 

 seen, with contractility, then contract. That action of the nerve-fibres 

 of certain nerves, which consists in conducting the effects of stimuli 

 to the muscular tissue, and causing it to contract, was named by Hal- 

 ler the vis nervosa ; and there is reason to believe that the property, 

 by which other nerves conduct the effects of stimuli inwards to the 

 nervous centres, is of precisely the same nature. This power of con- 

 ducting the effects of impressions, in either direction, may be named 

 conductility, or, as has been suggested, neurility ; whilst the general 

 term excitability must include, not only this conductility, but likewise 

 the power of receiving impressions, possessed by the nerves and ner- 

 vous centres, and also the special reactions and actions of the latter, 

 whether these be sensorial or motorial. 



The general organizing or formative property is that on which the 

 development, growth, and nutrition or maintenance, of all the animal 

 tissues depend. It is also a complex property, or may, at least in 

 imagination, be supposed to consist of two associated properties. 

 One of these is purely assimilative, and enables a tissue to appropriate 

 to itself such external matter as it needs, and to convert it into its 

 own substance, for the purposes of its increase, whilst undergoing de- 

 velopment and growth ; or for its maintenance during those conditions 

 of waste and renovation, which accompany and follow the exercise or 

 use of the tissue. This purely assimilative or nutritive property is 

 sometimes named the metabolic property or vital affinity. The second 

 organizing property modifies or controls the direction of the assimila- 

 tive property, so far as to guide its operation to the production of 

 certain organic forms, both in the entire individual, and in the sepa- 

 rate organs, parts, tissues, and ultimate structural elements of which 

 it consists. This is the proper organizing or plastic property, and 

 has been also named the metamorphic property. The metabolic or 

 assimilative process is evidently a chemical process of a higher char- 

 acter than ordinary chemical processes; or as it may be termed, a 

 vito-chemical process ; but the metamorphic or plastic property is 

 purely and absolutely a vital process. 



Of the three .general vital properties just described, the last, or 

 organizing property, is common to plants and animals. An imperfect 

 form of contractility occurs in a few parts of certain plants; but sen- 

 sibility, or nervous excitability, is quite peculiar to animals. 



These vital properties of the animal tissues seem to be, as it were, 

 dormant in the living organism, whether it be a germ, embryo, or 



