COMPARATIVE ANATOMY AND PHYSIOLOGY. 



And how can that consciousness and will, which are one in each individual, 

 exist in so many unconnected particles ? If, then, we allow any sensibility, 

 consciousness, and voluntary power, to the beings of this group of Acrita to 

 deny which would be in effect to exclude them from the Animal Kingdom 

 we must regard these faculties as associated with nervous filaments, of such 

 delicacy as to elude our means of research. When the general softness of 

 the textures, and the laxity of structure that characterize the nervous fibres, 

 in the lowest animals in which they can be traced, are kept in view, little dif- 

 ficulty need be felt in accounting for their apparent absence. The case is very 

 different from that of Vegetable structure, the greater consistency of which 

 enables us to place much more reliance upon the negative evidence afforded 

 by anatomical research. 



129. The correctness of this view (which has been here dwelt on the longer, 

 because it involves a fundamental question in Nervous Physiology), is borne 

 out by the fact, that, in those members of the group whose size and consistency 

 allow their structures to be sufficiently examined, a definite nervous system 

 has been detected, in the position which it might, a priori, be expected to 

 occupy, according to the type of the individual. Thus, in the large fleshy 

 isolated polype, commonly known as the Sea-Anemone (Jlctinia), a nervous 

 ring has been discovered, surrounding the mouth as in other Radiata, and 

 sending off branches to the tentacula, with a minute ganglionic enlargement 

 at the base of each. In the higher Radiata, as the Star-Fish, the nervous 

 system has the same regular form as that which prevails through the other 

 organs. The mouth is surrounded by a filamentous ring, which presents a 

 regular series of gangliohic enlargements, one of them corresponding with 

 each segment of the body. From every one of these, a branch is transmitted 

 to the corresponding ray ; and two smaller ones proceed to the viscera in- 

 cluded in the central disk. 



180. The Polypiftra being the lowest of the Radiated classes in w r hich 

 there is a regularly-organized digestive apparatus, and which perform move- 

 ments of a character ascribable only to a nervous system, it will be desirable 

 to inquire a little more particularly into the phenomena they exhibit, and the 

 degree in which these necessarily involve the possession of the higher mental 

 endowments. In this inquiry we shall refer principally to the little Hydra, 

 or fresh-water Polype, the habits of which are better known than those of any 

 other species. Although no nervous filaments have been detected in this, we 

 have a right to infer their presence for the reasons already given ; and they 

 probably form a ring around the mouth, as in the Actinia, sending filaments to 

 the tentacula. This interesting little being may be regarded as essentially a 

 stomach; and the orifice of this is provided with tentacula, which contract 

 when irritated by the touch of any adjacent body, and endeavour to draw it 

 towards the entrance. Now, the action in the Human body, to which this is 

 most allied, is evidently that of the muscles of Deglutition ; which lay hold, as 

 it were, of the food that has been conveyed to the fauces, and carry it into the 

 stomach. These muscles are called into action, not by an effort of the will, 

 but by the contact of the food with the lining membrane of the pharynx. This 

 impression is propagated by the glosso-pharyngeal nerve to the medulla 

 oblongata, where a respondent motor impulse is excited, which is transmitted 

 through the pharyngeal branches of the par vagum to the muscles of degluti- 

 tion, and causes their contraction. This phenomenon will be more fully ex- 

 amined hereafter ; it is here adduced simply as an instance of the important 

 class of reflex movements which are independent of the brain (though, to a 

 certain extent, controlled by it), which are altogether involuntary, and which 

 do not necessarily involve the production of sensation. There would appear 

 to be little difference, in the character of this movement, between the simple 



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