202 LECTURE XVI. 



ciple of irrelative repetition prevails in the nervous system as in the 

 skeleton and locomotive instruments, yet it does not prevent the re- 

 cognition of the leading physiological divisions of that system. We 

 have, for example, the supra-oesophageal or cephalic portion, which is 

 subservient to the functions of the special organs of the senses, and is 

 the centre whence voluntary impulse may be directed along the non- 

 ganglionic tracts of the nervous axis, and to which ordinary sensa- 

 tion may be transferred by similarly uninterrupted nervous filaments. 

 We have, secondly, a large sub-cesophageal mass, which, originating the 

 nerves analogous to the fifth pair, for the masticating organs and 

 other parts of the head, maybe regarded as analogous to the medulla 

 oblongata. In the abdominal chords and ganglions we have the re- 

 quisite machinery for the automatic reception and reflexion of stimuli, 

 independently of sensation and the will ; and to these are superadded 

 internuntiate and uninterrupted chords, for bringing the body under 

 the dominion of the will, and for producing harmony and consent of 

 action throughout its extent. The special nerves to the respiratory 

 system, and the stomato-gastric nerves, complete this already compli- 

 cated nervous system. 



In the hexapod insects the nervous system diflfers chiefly from that 

 in the Myriapods in having its primary divisions more definitely de- 

 veloped, and in manifesting degrees of concentration corresponding 

 with the increase of bulk and strength in particular parts of the trunk, 

 and in the locomotive organs appended thereto. Most insects, how- 

 ever, commence their career as worms ; the high form which they are 

 ultimately destined to attain in the articulate series is at first masked by 

 the guise of an Anellide or Entozoon. Some insects retain their larval 

 or vermiform state much longer than others ; and after passing a great 

 proportion of their lives under this form, fall into the state of the pupa, 

 or chrysalis, relapsing, as it were, a second time into the condition of 

 an ovum, there and then undergoing that part of their development 

 which before was left incomplete, and finally, emerge in their perfect 

 state to enjoy for a brief period the highest faculties, animal and 

 organic, which they are destined to acquire; fluttering in the air, it 

 may be, for a single day, procreating their kind, and perishing. Now 

 the development of the nervous system, like that of the muscular, 

 digestive, and other systems, being completed at distinct and some- 

 times remote periods, requires to be studied in the first and last of the 

 active states of the insect, and also in the intermediate period, when, 

 owing to the rapidity and extent of the changes which it undergoes, 

 the nervous system offers to the comparative anatomist and physiologist 

 phenomena of the highest interest. 



The apodal Entozoiform larvae, in which the segments of the body 



