DEVELOPMENT OF THE TEXTURES. ix 



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 which are distributed to them. Hence it is lost in parts severed 

 from the body, and it may be immediately 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 not improbable 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 originated 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 conduct sensorial impressions 

 and the before-mentioned " vis nervosa " are one and the same vital property ; the 

 difference of the effects resulting 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 OF THE TEXTURES. 



The tissues of organised bodies, however diversified they may ultimately 

 become, show a wonderful uniformity in their primordial condition. From 

 researches which have been made with the microscope, especially during the 

 last few years, it has been ascertained that the different organised structures 

 found in plants, and to a certain extent, also those of animals, originate by 

 means of minute corpuscles, which, having for the most part a vesicular 

 structure, have been named cells. These so-called cells, remaining as sepa- 

 rate corpuscles in the fluids, and grouped together in the solids, persisting 

 in some cases with but little change, in others undergoing a partial or 

 thorough transformation, produce the varieties of form and structure met 

 with in the animal and vegetable textures. Nay, the germ from which an 

 animal originally springs, so far at least as it has been recognised under a 

 distinct form, appears as a cell ; and the embryo, in its earliest stages, is 

 but a cluster of cells produced apparently from that primordial one ; no 

 distinction of texture being seen till the process of transformation of the 

 cells has begun. 



No branch of knowledge can be said to be complete ; but, even now that 

 a quarter of a century has elapsed since the promulgation of the cell- 

 doctrine, there is, perhaps, none which can be more justly regarded as in a 

 state of progress than that which relates to the origin and development of 

 the textures, and much of the current opinion on the subject is uncertain, 

 and must be received with caution. In these circumstances, in order both 

 to facilitate the exposition, and to explain to the reader more fully the 

 groundwork of the doctrines in question, we shall begin with a short 



