832 SPECIAL PHYSIOLOGY. 



common sensation from all parts of the body and limbs. Afferent and effer- 

 ent fibres likewise end in and spring from these ganglia, proceeding to the 

 head, the segmented trunk, and the limbs, and officiating in the various and 

 extraordinary consensual, instinctive movements exhibited by the highest of 

 these animals, especially by the spiders, ants, and bees, in the construction of 

 their webs, nests, and cells, and in the volitional acts implied in any special 

 movements, particularly when they are subjected to unusual or opposing cir- 

 cumstances. 



The series of ganglia peculiar to these animals, which are connected to- 

 gether, forming the double ganglionated cord found on the abdominal aspect of 

 their segmented bodies, are the locomotive ganglia, corresponding in function 

 with the pedal ganglia of the Mollusca ; they constitute the excito-motor 

 reflex centres for the locomotive acts, which, in these animals, as in the vari- 

 ous insects, spiders, and myriapods, and even in the swimming Crustacea, are 

 probably essentially automatic, and performed independently of sensation, 

 though they may be associated with it, and are independent of volition, al- 

 though they may often be controlled by it. The ascertained structure of this 

 ganglionated nervous cord, corresponds entirely with these combined func- 

 tions ; for some of the fibres of the roots of the nerves which arise from it, 

 are seen to end in the gray matter of the ganglion of their own segment, or 

 to pass out at the opposite side, or at the same side, often becoming, as de- 

 monstrated in the leech, connected with processes from the nerve cells ; other 

 fibres pass from the nerves of one segment up and down, along the cord, 

 through one, two, or even three adjacent ganglia, and then pass out into as 

 many corresponding nerves of the same or of the opposite side, above and 

 below ; other fibres proper to the cord, act as short longitudinal commissural 

 fibres, uniting the ganglia of adjacent segments, and joining the first gangli- 

 onic masses to the cephalic ganglia, those cords which pass by the oesophagus 

 being compared to the medulla oblongata ; lastly, fibres are met with some- 

 times, named transcurrent, which pass over the several ganglia, and form lon- 

 gitudinal tracts, extending upwards to the cephalic ganglia. By these last- 

 named fibres all parts of the system are brought into subjection to the chief 

 or cephalic apparatus ; whilst, within itself, every segment with its ganglion 

 and nerves can act, either independently or in combination with other seg- 

 ments. These anatomical facts present a sort of analysis of the arrangements 

 believed to exist in the more complex spinal cord of the Yertebrata, which is 

 supposed to consist of independent centres, fused together by continuity of the 

 gray matter ; functionally and structurally we here recognize a homology, 

 though, as already mentioned, there is, as yet, no evidence that the double 

 ganglionated cord of the Annulose type is the anatomical homologue of the 

 spinal cord of the Vertebrate type. Experiments have demonstrated most con- 

 clusively that this part of the nervous system of an Annulose animal, consists 

 of independent and purely reflex centres, and that they yield phenomena pre- 

 cisely similar in character to, but even more striking man, those presented by 

 lizards, frogs, and newts. Thus, a decapitated insect, nay, even a single seg- 

 ment of a centipede, continues to perform symmetrical and characteristic 

 movements when it is irritated, or placed in such a position as to be stimu- 

 lated to action ; a water-beetle, for example, if beheaded and then placed in 

 water, will perform natatory movements. When, however, portions of an 

 Annulose animal, severed from their connection with the cephalic ganglia, are 

 left untouched, or are not subjected to any special stimulus, they remain qui- 

 escent and immovable. Moreover, though a decapitated centipede will, if 

 irritated, continue an onward movement and push its headless trunk against 

 any opposing body, it will not mount over it, turn aside, or move backwards, 

 as it would do if still under the guidance of sensation and a low form of voli- 

 tion ; it cannot adapt or suit its movements to the nature of the obstacle or 

 impediment placed in its path. The multiplied feet of these animals demand 

 a corresponding multiplication of the ganglionic reflex excito-motor centres, 

 but they are made to act harmoniously, in succession and in alternation, on 

 the two sides, which pass from nerve to nerve, and from one segment of the 

 cord to another ; whilst, in the perfect animal, all are brought into harmony, 

 either instinctively or volitionally, by the transcurrent cephalic fibres. The 



