MUSEUM OF COMPARATIVE ZOOLOGY. 169 



The internal structure of the brain shows a strictly bilateral arrange- 

 ment of the elements. A transverse section through the middle of the 

 ganglionic mass is represented in Figure 24 (Plate III.). The fibrous 

 matter is collected into two commissural masses, in which the fibres run 

 both anteroposteriad, chiefly at the lateral extremities, and laterally, 

 chiefly in the middle. The real relation of these commissures to each 

 other is first seen in sagittal sections (Plate II. Figs. 19 and 20), where 

 the fibrous matter has the form of a > with the apex directed forward. 

 The dorsal arm of this > is prolonged backwards in two lateral horns, 

 which are surrounded by ganglionic cells. The tips of these horns, cut 

 transversely, are seen in Figure 25 (Plate III). The similar ventral 

 horns are the roots of the circumcesophageal connectives. From near the 

 anterior apex of the > a small arm of fibrous matter is directed forward, 

 as seen in a sagittal section of the brain near its left lateral margin (Plate 

 II. Fig. 19). This becomes, in a median sagittal section, a small commis- 

 sure cut transversely (corns, a., Fig. 20), and separated from the brain by 

 the connective-tissue capsule. This commissure is at its right side again 

 'connected with the brain, as already described, for the left extremity. 

 Thus it resembles in its form and relation to the main fibrous mass of 

 the brain the handle of a basket, the handle being directed forward. 

 It lies, as can be easily seen from the figures, immediately below the 

 surface of the cerebral organ, and its relation to that structure will be 

 more fully explained later. 



The arrangement of the ganglionic elements in the brain is somewhat 

 definite. Ganglionic cells of the first sort are found in nearly every 

 part, and make up all diffuse centres, where, however, transitional forms 

 render their separation from the second class difficult. The former are 

 most strongly marked at the tip of the dorsal horn (cl. gn. I., Fig. 25), 

 where they are very densely crowded. They cover also the lateral and 

 dorso-lateral aspects of the dorsal commissure (Fig. 24) in similar dense 

 masses. The anterior face of the fibrous matter is also almost exclu- 

 sively occupied by cells of the first class ; and from this region they 

 extend a short distance ventrally. Here one finds a gradual transition 

 into the ganglionic cells of the second class (cl.gn. IT., Plate II. Fig. 20, 

 Plate III. Fig. 24), which occupy the entire ventral and posterior as- 

 pects of the fibrous matter. These cells also fill the space between the 

 dorsal and ventral commissures, but are found dorsally only between 

 the two lateral fibrous swellings on the lateral edges of the dorsal 

 commissure. They are never so crowded as cells of the first class, and 

 display no particular arrangement into clusters or groups. 



