MKMOIKS or THE NATIONAL ACADEMV OF SCIENCES. 29'J 



\1. Tin: Mkiman Eye Ganci.ia on Lohes. 



d'l. I. I'igB. 5-7, H. I'iKM. H, it, 10; I'l. Ill, Fi({. 11, VII, Fi({». 1, 2.) 



After passiiip, iu liori/outiil sections, down tlirouKli the Iat<Tiil -eye nerves and lolies tlie 

 knife euts tlirougli the <irij;in of tlie niediiin eye nerves and the lobes or t,'an^'lia from wiiic li they 

 originate. 



These h>bea are, in the adult brain, situated considerably boh)\v the phine of tlic; lateral-eye 

 lobes, and the origin of the bundle of nerve-libers leading to the median eye nerve is diflicnlt to 

 detect, owing to the fact that, so far as our sections show us, they are small, slendei-. (|uite irreg- 

 ular in shape, not being regularly pear-shajjed or subspherical, uiul their (ibers api)ear to be more 

 or less continuous with the iil)ers of the ct^rebral lobes; in fact, they at first sight appear to be 

 branches of the inner jwrtion 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 

 them.selves, 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 shoNvn the mode of 

 origin of the libers fornung the origins or roots of the nerves, from any large ganglinu-cells. Tlie 

 ganglia are more or less blended or t-oalesced with the cerebral ganglia, and I lielicvc it will be 

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

 first undt, or w hen 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 appears 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 

 gitng., PI. Ill, 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)assed through 

 the middle of the median eye nerve (w. eyen.). The nerve fibers from it pass in towards the middle of 

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

 or offshoots ol' the fibrous portion of the lateral-eye lobes. The triangular space between these 

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

 (c. eom.). 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 from any of these ganglion-cells entering into the cerebral lobes, I takeit that the cells in 

 (piestlon 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, recnforced with fibrilhe 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 what seems the right root of the 

 nu'dian eye nerve is seen to penetrate farther into the center of the brain than iu 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 colls nearly meet 

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

 Fig. 14, there are no traces of the median eye nerve, the rulUc-Iike 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 PJ. VII, Figs. 1 and li. In Fig.l is seen the horseshoe shaped bundle of fibers, inclosing the 



•The limits of the fihrous portion of theac lobos have lieen clearly distinguished by Viallanes (p. 116 and his V\. 

 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 sec that my I'l. VII, Figs. 1 and 2 represents the ganglia in question. It is the blending or fusion of the portions 

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

 a clear idea of their limits. In this re8|)ect the brain of Limulus differs remarkably from that of Crustacea or 

 Insects, as well as Arachnida. 



