MORPHOLOGY OF THE BRAIN 227 



direct continuity in the fibres forming the two main longitudinal 

 commissures of the nervous cord, and which originate in the brain, 

 it seems to follow that the movements of the body are in large part 

 directed or coordinated by the brain. 1 Still, however, a second 

 brain, so to speak, is found in the third thoracic ganglion of the 

 locust, which receives the auditory nerves from the ears situated in 

 the^base of the abdomen; or in the first thoracic ganglion of the 

 green grasshoppers (katydids, etc.), whose ears are situated in their 

 fore legs ; while even the last pair of abdominal ganglia in the cock- 

 roach and mole cricket, is, so to speak, a secondary brain, since it 

 distributes sensory nerves to the caudal stylets, which are provided 

 with organs probably olfactory in nature. 



It is impossible to understand the morphology of the brain unless 

 we examine the mode of origin of the nervous system in the early 

 life of the embryo. The head of an embryo insect consists of six 

 segments, i.e. the ocular, antennal, premandibular, mandibular, and 

 the 1st and 2d maxillary segments, so named from the appendages 

 they bear. Of these the first three in the larva and adult are 

 preoral, and the last three are postoral. The antennal segment 

 was probably either postoral in the progenitors of insects, or the 

 antennae were inserted on the side of the mouth, the latter finally 

 moving back. 2 



The nervous system in the early embryonic condition, as shown 

 by Wheeler (Fig. 245), at first consists of nineteen pairs of primitive 

 ganglia, called neuromeres. Those of the head, which later in em- 

 bryonic life fuse together to form the brain, are the first three, 

 corresponding to the protocerebrum, deutocerebrum, and tritocerebrum 



ously from oiie end of the ventral or nervous cord to the other. " The commissures 

 take their origin neither out of a central punctsubstauz (or marksubstanz), nor 

 from the peripheral ganglion-cells of the several ganglia, but are mere continuations 

 of the longitudinal fibres which decrease posteriorly in thickness, and extend ante- 

 riorly through the commissures, forming the oesophageal ring, to the brain." 



1 The following extract from Newton's paper shows, however, that the infra or 

 suboesophageal ganglion, according to Faivre, has the power of coordinating the move- 

 ments of the body; still, it seems to us that the brain is primarily concerned in the 

 exercise of this power, as the nerves from the suboesophageal ganglion supply only 

 the mouth-parts. "The physiological experiments of Faivre in 1857 (Ann. des Sci. 

 Nat. torn, viii, p. 245), upon the brain of Dyticus in relation to locomotion, are of 

 very considerable interest, showing, as they appear to do, that the power of coordi- 

 nating the movements of the body is lodged in the infraresophageal ganglion. And 

 such being the case, both the upper and lower pairs of ganglia ought to be regarded 

 as forming parts of the insect's brain." Quart. Jour. Micr. Sc., 1879, p. 342. 



2 The arthropod protocerebrum probably represents the annelid brain (supra- 

 cesophageal ganglion). The antennal segment (deutocerebrum), with the preman- 

 dibular (intercalary) segment (tritocerebrum) originally postoral, have, as Lankes- 

 ter suggests, in the Arthropoda moved forward to join the primitive brain. See 

 Wheeler, Journ. Morphology, Boston, viii, p. 112. 



