398 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



principally sensory. One pair arise from the oro-lateral 

 (ventral) border of the cord (yn. nv.), and furnish principal 

 branches to the interarticular muscles, numerous twigs 

 which end in sensory cells of the external epithelium, 

 and branches which are said to unite with similar ones 

 from the lateral brachial cords of the deeper oral system. 

 The other pair arise from the dorso-lateral border of the 

 cord (dr. nv.), and the numerous branches divide again 

 and again until, as exceedingly fine twigs, they end 

 like some of those of the oral pair, in continuity 

 with the sensory cells of the external epithelium. 

 In longitudinal sections of the arms (PL V., fig. 50) 

 the brachial nerve cord (br. nv. c.) presents a sort of 

 nodal enlargement between every two joints, those between 

 which a syzygy occurs excepted. These are the points of 

 origin of nerves which are distributed to the interbrachial 

 muscles and ligaments, and which are exceedingly difficult 

 to trace. 



In histological structure the central capsule and its 

 radial cords are practically uniform. They consist of 

 extremely delicate fibrils, with numbers of very minute 

 ganglion cells intercalated between them. In the more 

 apical portion of the central capsule the fibrils have no 

 definite arrangement, but in the walls of its oral aspect 

 they are disposed for the most part concentrically around 

 the chambered organ (fig. 55). In the radial cords the 

 nerve fibrils do not form one homogeneous bundle, but are 

 bound together to form strands which are definite in 

 position and direction. Thus, the chiasma (PI. VI. , fig. 53, 

 chi.) is formed by two strands which run along the outer 

 lateral borders of the undivided cord, and from thence 

 cross over, one under the other, to be merged into similar 

 strands which run along the inner borders of the two 

 branches and, by the union of their proximal ends, form 



