MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 299 



VI. The Median-Eye Ganglia ok Lobes. 



(PI. I, Figs. 5-7, II, Figs. 8, 9, 10; PI. Ill, Fig. 11, VII, Figs. 1, 2.) 



After passing, in horizontal sections, down through the lateral-eye nerves and lobes the 

 knife cuts through the origin of the median-eye nerves and the lobes or ganglia from which tliey 

 originate. 



These lobes are, in the adult brain, situated considerably below the plane of the lateral-eye 

 lobes, and the origin of the bundle of nerve-fibers leading to the median-eye nerve is diflBcult to 

 detect, owing to the fact that, so far as our secfious show us, they are small, slender, quite irreg- 

 ular in shape, not being regularly pear-shaped or subspherical, and their fibers appear to be more 

 or less continuous with the fibers of the cerebral lobes ; in fact, they at first sight appear to be 

 branches of the inner portion of the fibrous matter of the cerebral lobes. The two median-eye 

 nerves being blended, within the brain as well as without, only dividing near the median eyes 

 themselves, their cortical cells are also near together and not separated into distinct areas. I 

 have had no sections either of the young or adult animal which have clearly shown the mode of 

 origin of the fibers forming the origins or roots of the nerves, from any large ganglion-cells. The 

 ganglia are more or less blended or coalesced with the cerebral ganglia, and I believe it will be 

 diflicult, if not impossible, to make them out unless we examine sections of the young after the 

 first molt, or when about half an inch in length, exclusive of the caudal spine.* 



I have been able to trace the origin of the median-eye nerve into the central part of the brain 

 to the point m. eye n. in PI. Ill, Fig. 11, and it ajipears probable that the large ganglion-cells 

 which give origin to a part of the median-eye nerve are associated more or less intimately with 

 those which supply the fibers, or a part of them, to the inner aspect of the cerebral lobes. We 

 will, however, consider the large ganglion-cells, to be seen in the section represented at m. eye 

 ganf/., PL III, Fig. 11, as belonging to the median-eye lobe. This area of large ganglion-cells, 

 with smaller ones intermingled, is bilateral, the cells being arranged irregularly on each side of an 

 irregular and very slightly marked median line which is clear of them. 



We will now describe the sections passing through the blended median-eye nerves, and the 

 double lobe from which the double nerve originates. In PI. II, Fig. S, the knife has i)asscd through 

 the middle of the median-eye nerve {m. eye n.). The nerve fibers from it pass in towards the middle of 

 the brain and partially fuse with two masses (PI. II, Fig. 9, m. eye I.) which appear as two horns 

 or offshoots of the fibrous portion of the lateral-eye lobes. The triangular sjiace between these 

 two roots of the median-eye fibrillar masses is bounded ])osteriorly by the cerebral commissure 

 (c. com.). The ganglion-cells scattered through this mass are arranged on each side of the 

 obscurely-marked median line of the brain. As in this plane of the brain I can perceive no 

 fibers fi'om any of these ganglion-cells entering into the cerebral lobes, I take it that the cells in 

 question send their fibers into the median-eye lobes and help build up the median-eye nerve. 

 The fibers of the latter are also, with little doubt, reenforced with fibrillie from the masses of 

 chromatic or small ganglion-cells which envelop the nerve just before it passes out of the brain. 



In PI. Ill, Fig. 11, the cerebral commissure is complete, and wliat .seems the right root of the 

 median eye nerve is seen to penetrate farther into the center of the Ijrain than in the two sections 

 immediately above. In PI. Ill, Fig. 12, the knife has passed just below the roots of the median 

 eye nerve and the cylindrical plates or sheets of small ganglion or chromatic cells nearly meet 

 under the nerve. This is also seen in PI. Ill, Fig. 13, while in the section represented by PI. Ill, 

 Fig. 14, there are no traces of the median eye nerve, the ruffle-like cylinders of small ganglion 

 cells being now crowded together under where the nerve passes out of the brain. 



The relations of the median-eye lobes are well shown in the sections of a small Limulus, repre- 

 sented by PI. VII, Figs.l and 2. In Fig. 1 is seen the horseshoe shaped bundle of fibers, inclosing the 



*Th6 limits of the fibrous portion of these lobes have been clearly distinguished by Viallanes (p. 416 and his PI. 

 II, Figs. 3 and 5, goc, though in his model he does not represent the central mass of large and small ganglion cells. 

 I now see that my PI. VII, Figs. 1 and 2 represents the gauglia in question. It is the blending or fusion of the portions 

 of these and the lateral-eye lobes adjacent to the cerebral lobes, which has rendered it so diiBcult for me to obtain 

 a clear idea of their limits. In this respect the. brain of Limulus differs remarkably from that of Crustacea or 

 Insects, as well as Arachnida. 



