218 Miscellaneous. 



CBsophageal or posterior centres of the Gasteropoda in general, and 

 of the aquatic Pulmonata in particular, of regions, lobes, or lobules 

 having a peculiar structure, constant connexions, and of course di- 

 stinct and precise physiological attributes. 



The anterior centre of the oesophageal collar is destined to inner- 

 vate the foot — that is to say, the immediate organ of movement, a 

 motor organ which nevertheless is endowed with great sensibility. 

 Now minute anatomy shows clearly in Paludina (a species of 

 another group) that a thick cord descends from the posterior or 

 cerebroid centre, attaches itself to the connective uniting the brain 

 and the pedal ganglion, and gives origin to the nerve destined for the 

 superior and eminently sensitive portion of the skin of the foot. Is 

 it possible, after this observation, to refuse to recognize that the 

 anterior ganglia are evidently in relation to motivity, and that the 

 posterior centres are more particularly connected with sensibility ? 

 and, lastly, that these latter send forth to the other ganglia the 

 fibres destined to give them sensibility ? 



From these facts we see how incomplete, from a physiological 

 point of view, was the knowledge that we possessed with regard to 

 the centres of innervation, and how important it was, by minute 

 analysis, to arrive at the distinction of the special secondary parts in 

 these centres. 



This remark acquires still more interest when we ascertain, as I 

 have done, that the more the nerves are destined for the perception 

 of delicate and subtle impressions (in a word, the more they are 

 sensitive and specialized), the more also are their origins placed upon 

 the posterior face of the subcesophageal cerebroid centre. 



It is not sufficient therefore to indicate in a general way what are 

 the nerves that spring from a ganglion ; it is also necessary to seek 

 the points where, relatively and absolutely, the real origins occur. 

 This is so true that cutaneous branches have been described as 

 issuing from the optic nerve, which itself, in some cases, seems to 

 spring from the nerve of the tentacle. This fact, which is difficult 

 to admit a priori (since nothing analogous is met with in the 

 higher animals, the nervous system of which is so weU known), is 

 not proved by minute histological researches. On the contrary, on 

 separating, by histological processes and reactions, the bundles of 

 fibres simply laid together and approximated under the same general 

 neurilemma, we may ascend, in Physa, Planorbis, Lymncea, &c., to 

 the real, precise, constant, and always distinct origin of the three 

 nerves of special sensibility, the oLfactive, acoustic, and optic nerves, 

 and thus prove that this origin is always met with a little laterally 

 upon the posterior face of the subcesophageal centre, round a small, 

 prominent, hemispherical lobule, which merits the name of the 

 lobule of special sensibility. 



By indicating with precision the isolation and the distinct starting- 

 points of the nervous cords going to the three principal organs of the 

 senses, I bring a new proof in support of the general idea that, in 

 the lower animals, in the nervous centres hitherto regarded as simple 

 3.nd homogeneous, there exist secondary parts which it is necessary 



