COMPARATIVE ANATOMY AND PHYSIOLOGY. 107 



illustrated in the curious disposition of parts which we find in the arms of the 

 Cuttle-fish. These are provided, it is well known, with a series of suckers, 

 which are to the animal important instruments of locomotion and prehension. 

 It has been observed by Dr. Sharpey, that the nerves which supply these arms 

 are furnished with ganglionic enlargements, of which one corresponds with 

 each sucker: and that each trunk consists of two tracts, in one of which the 

 ganglionic enlargements exist ; whilst the other passes continuously over these, 

 but sends off nervous filaments, which help to form the branch going to each 

 sucker. It has been supposed that the white or fibrous tract is the motor por- 

 tion, and the ganglionic the sensory ; but this is inconsistent with the facts 

 known, regarding the influence of the nerves upon the movements of the 

 suckers. When the animal wishes to embrace any object firmly with its arm, 

 it brings all the suckers simultaneously to bear upon it. There can be little 

 doubt that this action is occasioned by a motor impulse, propagated from the 

 cephalic masses by the non-ganglionic portion of the cord, which supplies all 

 the suckers alike. On the other hand, any individual sucker may be made 

 to attach itself, by placing a substance in contact with it alone ; this action is 

 independent of the cephalic ganglia, as is evident from the fact that it will 

 take place when the arm is severed from the body, or even in a small piece of 

 the arm, if recently separated ; and it can scarcely be doubted, that it is due 

 to the reflection of the impression made upon the sucker, through the small 

 ganglion in its neighbourhood, where it excites a motor impulse. The operation 

 of these independent centres appears, in the entire living animal, to be con- 

 trolled, directed and combined, by the cephalic ganglia, through the medium 

 of the fibrous band which passes over them, and which mixes its branches with 

 theirs. A very similar arrangement will be presently shown to exist in the 

 double nervous column of the Articulata. 



140. Upon reviewing all the anatomical facts hitherto stated, it will be per- 

 ceived that ganglionic masses, characterized by nuclei, of gray matter, or of 

 something equivalent to it, seem to exist, wherever it is desirable that impres- 

 sions made upon the afferent nerves should excite motions ; and that, as we 

 rise in the scale, there is an increase in the number of centres possessing a 

 diversity of functions. We have seen that sometimes these centres are, for 

 the sake of convenient disposition, united into one mass ; whilst, on the other 

 hand, when the organs are multiplied, they also are repeated to a like extent ; 

 especially when it is desirable that they should be able to act independently 

 of one another, as in the case of the suckers of the Cuttle-fish. It may further 

 be remarked, that, wherever the presence of special sensory organs, confined 

 to one part of the body, gives to that part a predominance over the remainder 

 (the entrance to the alimentary canal being always in this neighbourhood), we 

 find the ganglia with which they are connected possessing a special relation 

 with all the rest, which these do not possess with each other. It is obvious 

 that, where visual organs are developed, the impressions made upon these will 

 determine the movements of the animal more than those of any other kind; 

 and it would seem to be chiefly owing to the information they communicate, 

 that the cephalic ganglion has such an evident presiding influence over the 

 rest, even when smaller than any of .them. This is, however, more the case 

 in animals whose movements are rapid, and in which, therefore, the perception 

 of distant objects is more important as in the Insect tribes. Except in the 

 Cephalopoda, the subservience of the nervous system to the nutritive functions 

 of the Mollusca is so great, that it might almost be regarded as an appendage 

 to the digestive organs, destined for the selection and prehension of aliment. 

 But in the more active members of that class, it derives a more elevated cha- 

 racter, from the development of organs of special sensation and of active loco- 

 motion. 



