890 ANATOMY AND DEVELOPMENT 



Sheath of the Ventral Cords. The ventral cords are en- 

 veloped by a double sheath, the two layers of which are often in 

 contact, while in other cases they may be somewhat widely 

 separated from each other. The inner layer is extremely thin 

 and always very closely envelopes the nerve-cords. The outer 

 layer is thick and fibrous, and contains a fair sprinkling of 

 nuclei. 



Supra-cesophageal Ganglia. In the present state of our know- 

 ledge a very detailed description of the histology of the supra- 

 oesophageal ganglia would be quite superfluous, and I shall 

 confine myself to a description of the more obvious features in 

 the arrangement of the ganglionic and fibrous portions (PI. 50, 

 fig. 19 A G). 



The ganglion cells are in the first place confined, for the 

 most part, to the surface. Along the under side of each gan- 

 glion there is a very thick layer of cells, continuous behind, 

 with the layer of ganglion cells which is placed on the under 

 surface of the cesophageal commissures. These cells have, 

 moreover, an arrangement very similar to that in the ventral 

 cords, so that a section through the supra-cesophageal ganglia 

 has an obvious resemblance to what would be the appearance 

 of a section through the united ventral cords. On the outer 

 borders of the ganglia the cells extend upwards, but they end 

 on about the level of the optic nerve (fig. 19 D). Immediately 

 dorsal to this point the fibrous matter of the brain is exposed 

 freely on the surface (fig. 19 A, B, &c., a}. I shall call the region 

 of fibrous matter so exposed the dorso-lateral horn of white 

 matter. 



Where the two ganglia separate in front the ganglion cells 

 spread up the inner side, and arch over so as to cover part of 

 the dorsal side. Thus, in the anterior part, where the two 

 ganglia are separate, there is a complete covering of ganglionic 

 substance, except for a narrow strip, where the dorso-lateral 

 lobe of white matter is exposed on the surface (fig. 19 A). From 

 the point where the two ganglia meet in front the nerve-cells 

 extend backwards as a median strip on the dorsal surface (fig. 

 19 D and E). This strip, becoming gradually smaller behind, 

 reaches nearly, though not quite, the posterior limit of the junc- 

 tion of the ganglia. Behind it there is, however, a region where 



