6 DEYELOrMEXT OF THE TEXTURES. 



Mechanical irritation, as scratching ^vith a sharp point, or slightly 

 ;)inchin"- with the forcei^s, electricity obtained irom a piece of copper 

 and a p?ece of zinc, or from a larger apparatns if necessary, and the 

 '^ndden ain>licatioii of cold, are the stimuli most commonly applied. 

 "^' 3 Vis Nervosa. — Tlie stimulus which excites contraction may be 

 applied either directly to the muscle, or to the nerves entering it, which 

 then communicate the eifect to the muscular fibre, and it is in the 

 latter mode tliat the voluntary or other mental stimuli are transmitted 

 to muscles from the brain. Moreover, a muscle may be excited to con- 

 tract by irritation of a nerve not directly connected with it. The 

 stimulus, in this case, is first conducted by the nerve irritated, to the 

 brain or spiual cord ; it is then, Avithout participation of the will, and 

 even without consciousness, transferred to another nerve, by which it is 

 conveved to the muscle, and thus at length excites muscular contrac- 

 tion. ' The property of nerves by which they convey stimuli to muscles, 

 whether directly, as in the case of muscular nerves, or circuitously, as 

 in the case last instanced, is named the " vis nervosa." 



4. Sensibility. — AVe become conscious of impressions made on 

 various parts of the body, both external and internal, by the faculty of 

 sensation ; and the parts or textures, impressions on which are felt, 

 are said to be sensible, or to possess the vital property of " sensibility." 



This property manifests itself in very different degrees in different 

 parts ; from the hairs and nails, which indeed are absolutely insensible, 

 to the skin of the points of the fingers, the exquisite sensibility of 

 which is well known. But sensibility is a property which really depends 

 on the brain and nerves, and the different tissues owe what sensibility 

 they possess to the sentient nerves Avhich are distributed to them. 

 Hence it is lost in parts severed from the body, and it may be imme- 

 diately extinguished in a part, by dividing or tying the nerves so as to 

 cut off its connection with the brain. 



It thus appears that the nerves serve to conduct impressions to the brain, 

 which give rise to sensation, and also to convey stimuli to the muscles, which 

 excite motion : and it is probable that, in both these cases, the conductive 

 property exercised by the nervous cords may be the same ; the difference of effect 

 depending on this, that in the one case the impression is carried upwards to the 

 sensorial part of the brain, and in the other downwards to an irritable tissue, 

 which it causes to contract ; the stimulus in the latter case either having origin- 

 ated in the brain, as in the instance of voluntary motion, or having been first 

 conducted upwards, by an afferent nerve, to the part of the cerebro-spinal centre 

 devoted to excitation, and then transferred to an efferent or muscular nerve, 

 along which it travels to the muscle. If this view be correct, the power by which 

 the nerves condvict sensorial impressions and the before-mentioned " vis nervosa" 

 are one and the same vital property ; the difference of the effects resultmg from 

 its exercise, and, consequently, the difference in function of sensorial and motorial 

 nerves, being due partly to the different nature of the stimuli applied, but 

 especially to a difference in the susceptibility and mode of reaction of the organs 

 to which the stimuli are conveyed. 



DEVELOPMENT OP THE TEXTURES. 



The tissues of organised bodies, however diversified they may 

 ultimately become, show a wonderful uniformity in their primordial 

 condition. The results of modern researches have shown that the 



