LESSON 75.] NERVOUS SYSTEM IN INSECTS. 249 



sorted to, is on the same plane ; and lastly, the protection of the 

 bulk of the nervous system requires it to be transferred from the 

 dorsal to the ventral surface. 



1125. From its function of distributing the nerves of sense, there- 

 fore, the brain requires to be placed in the superior part of the head, 

 adjacent to the organs which it is destined to supply with nerves. 

 For a like reason (proximity to the structures to be supplied with 

 nerves), and, as stated in the preceding paragraph, for superior pro- 

 tection, the remainder of the nervous system is placed upon the ven- 

 tral surface of the body, and this could only be accomplished by di- 

 viding the brain, and transmitting the alimentary canal through the 

 space thus formed. 



1126. All the nerves which arise from the base of the brain are 

 motor in their function, and they are, therefore, distributed to the 

 muscles of the jaws. Nerves arise from the lower surface of the 

 sub-cesophageal ganglion, one on each side, which terminate in a 

 minute ganglion (c) called lateral ganglia ; from these a few delicate 

 nervous twigs arise, which are distributed to the muscles of the 

 throat. 



1127. The first thoracic ganglion (e) is near the base of the 

 brain ; its nerves are distributed to the muscles of the segment to 

 which it belongs. ** ^ 



1128. The second (/) and third (h) thoracic ganglia are remark- 

 able for their increased size, and the chords connecting them for 

 their great divergence. Between the loops thus formed a branch is 

 seen descending from the inferior surface of the ganglia respectively 

 (g, g), and giving off a branch right and left. 



1129. This structure is continued throughout the entire nervous 

 system, the branches being invariably given off above the ganglions. 

 By careful dissection their ultimate branches may be traced to the 

 muscles of the respiratory organs, and hence they have been thought 

 to constitute a distinct respiratory tract, combining a motor func- 

 tion. They are not such, however, but simply a system of arteries, 

 and these are the vessels which have been injected from the heart in 

 Insects, and in the Limulus, as already described. 



1130. The remainder of the nervous system is simply a repetition 

 of what has been already described, until we reach the terminal, en- 

 larged ganglion (&). This is called the caudal (tail) ganglion, and 

 is formed by the junction of the llth and 12th ganglions. 



1131. The most important portion of the nervous system of any 

 animal is the brain, and we present a much magnified view of that 



