FUNCTIONS OF THE CORD. 309 



t 



sition, the part in which the segmentation is first distinguished in insects 

 being the equivalent of the dorsal region in vertebrata, and that over 

 which the germinal membrane is last to close in, being homologous with 

 the ventral region. This view applies also to the position of the dorsal 

 vessel, which would then be on the ventral side of the axis, as in verte- 

 brata. Eegarded under this aspect, the longitudinal nervous tract of ar- 

 ticulata corresponds with the spinal cord of vertebrated animals in posi- 

 tion, as we shall find it does in function. 



" When the structure of the chain of ganglia is more particularly in- 

 quired into, it is found to consist of two distinct tracts, one of which is 

 composed of nerve fibres only, and passes backward from the cephalic 

 ganglia over the surface of all the ganglia of the trunk, giving off 

 branches to the nerves that proceed from them, while the other includes 

 the ganglia themselves. Hence, as in the mollusca, every part of the 

 body has two sets of nervous connections, one with the cephalic ganglia, 

 and the other with the ganglion of its own segment. Impressions made 

 upon the afferent fibres which proceed from any part of the body to the 

 cephalic ganglia become sensations when conveyed to the latter, while in 

 respondence to these, the consensual impulses, operating through the ce- 

 phalic ganglia, harmonize and direct the general movements of the body 

 by means of the efferent nerves proceeding from them. For the purely 

 reflex operations, on the other hand, the ganglia of the ventral cord are 

 sufficient, each one ministering to the actions of its own segment, and to 

 a certain extent, also, to those of other segments. It has been ascertained 

 by t"he careful dissections of Mr. Newport, to whom we owe all our most 

 accurate knowledge of the nervous system in articulated animals, that of 

 the fibres constituting the roots by which the nerves are implanted in 

 the ganglia, some pass into the vesicular matter of the ganglion, and, after 

 coming into relation with its vesicular substance, pass out again on the 

 same side (Fig. 150, /*, &), while a second set, after traversing the vesic- 

 ular matter, pass out by the trunks proceeding from the opposite side of 

 the same ganglion, and a third set run along the portion of the cord which 



connects the ganglia of different segments, 

 and enter the nervous trunks that issue from 

 them at a distance of one or more ganglia 

 above or below. 



"Fig. 150, from ganglionic tract of poly- 

 desmus maculatus. &, interganglionic cord ; 

 c, anterior nerves ; d, posterior ; f, Jc, fibres 

 of reflex action ; </, A, commissural fibres ; i, 

 longitudinal fibres, softened and enlarged as 

 they pass through the ganglionic matter. 

 Ganglion of poiydesmus macuiatna. " Thus it appears that an imrrassion con- 



