INSECTA. 



94T 



liv which it IB presumed tlicy arc thus endowed 

 willi voluntary power and sensation. Hut on 

 rcrlection it will appear tliat this ought really to 

 be the cave. To enable tlic IIIMT! In make a 

 forcible expiration and close its spiracle, lnch 

 is evidently an act of volition, the great con- 

 strictor of tdc spiracle ought to be endowed 

 wild voluntary nerves. On llic older hand, 

 MHO, as we know from experiment that the 

 insect has a voluntary power of closing, it must 

 also have a similar power of opening the orifice, 

 and, consequently, ihe retractor valvulae ought 

 to be supplied from tde same source as the 

 constrictor. The remaining portion of the 

 trunk of these nerves passes forwards and out- 

 wards, crosses the retractor of the spiracle 

 and gives off its third branch, which is again 

 divided, and sends one portion backwards to 

 the anterior (18) and the transverse abdominal 

 muscles (17), and the other forwards to the 

 transverse lateral (23). The remaining portion 

 of the nerve is distributed to the dorsal muscles 

 and teguments. The second nerve from the 

 gangliated part of the cord is much smaller 

 than tde first. It passes diagonally backwards 

 and outwards, and divides into two branches, 

 the first of wdicd is given to the latero-abdo- 

 minal muscles, and the second to the triangular 

 and transverse median, while the other (k) 

 passes downwards and outwards, and is con- 

 tinuous with part of the third branch of the 

 transverse nerves (i). 



Besides the nerves thus described as belong- 

 ing to tde moto-scnsitive cord in the thorax and 

 abdomen, there are others that merit particular 

 consideration, both from the circumstance of 

 their lying loosely above the cord, and from 

 their special distribution. These nerves, which 

 were formerly distinguished by us* as trtins- 

 n;',M- turrt-s from the direction of their prin- 

 cipal branches, and as respiratory from their 

 .special distribution to the respiratory organs, 

 were discovered by Lyonet, and are delineated 

 and particularly described in dis anatomy of 

 ('(>.* Itgmperdo. There is a plexus of them 

 in every Moment of the thorax and abdomen. 

 Like the alary nerves of the cord in the thorax, 

 there is a little difference in the distribution of 

 some of them in the Sphinx from that of the 

 (i in, ^ponding plexus in the Cossus. Incur 

 earlier examination! of these nerves-)- we be- 

 lieved them to originate from the posterior part 

 of each ganglion of the cord, and this also was 

 the opinion of Lyonet with reference to those 

 in the Cossus which constitute the second and 

 third plexus of the thorax, and the last of the 

 abdomen, and which, he expressly states, do 

 not come from tde cords, but from thenaiiL'lia J 

 We have since been satisfied tdat the plexus in 

 one segment is connected with that in each 

 succeeding one by means of a minute filament, 

 derived from the transverse portion of these 



Phil. Trans. 1832, part ii. p. 389, and 1834, 

 part ii. p. 401, also 1836, pan it. p. 544. 



t Op. oil. 183-2. 



J Traite Anat. de la Chenille, 1760. also 1782, 

 p. 98 and 204. 



nerves, and which, passing Literally over and 

 very close to the ganglion of the cord, ]' 

 fellow of the opposite side, in the middle Imp 

 behind il, ID fin ui tde lonjiiinlinal portion of 

 the in \t plexus, such filament gathering a few 

 additional ones from the upper or motor sur- 

 face of the cords. Hence, as we have stated,* 

 these nerves are of mixed character, and <"n- 

 tain some voluntary motor fibrils. Kadi plexus 

 is formed of these two filament-, wined, closely 

 approximated together, pass backwards along 

 the median line above the cord, until they 

 arrive just before the next ganglion, where they 

 diverge nearly at right angles, and are closely 

 approximated to another series of fibres that 

 runs in a commissural manner transversely 

 across the body, from one side to the oilier. 

 On each side a filament (c) is given off from 

 the transverse nerves to unite with the molo- 

 sensitive (J') close to the inner side of the 

 smaller rectus. Near ihe exlemal margin of 

 that muscle it gives off another branch 

 (g), which passes forwards upon the mus- 

 cle, unto which it gives filaments, and then 

 turns suddenly outwards (h), to join a branch 

 from the great moto-sensitive nerve, while a 

 smaller branch is continued onwards to supply 

 the remainder of the muscle. This union is 

 exceedingly interesting, and illustrates the fact 

 that, even in the Invertebrata, some of the 

 nerves in one part are connected by loops with 

 those in others, as noticed by physiologists in 

 the Vertebrated classes. The next branch (i) 

 of the transverse nerves is equally interesting 

 from the same circumstance. It is continuous 

 in the same manner with another branch of the 

 moto-sensitive (k). This branch is composed 

 of fibres that are approximated to the transverse 

 trunk, and pass some from without inwards, 

 and others from within outwards, to form the 

 nerve (i), leaving between them at its base a 

 little triangular interspace, covered by a mem- 

 brane and resembling the plexus (i). This 

 nerve passes directly forwards until it arrives 

 at the insertion of the greater recti (j), where 

 it gives off a large branch to those muscles, 

 and then passing beneath the oblique muscles, 

 unto which it is distributed, and to the trian- 

 gularis, becomes connected by loops with the 

 second pair of moto-sensitive nerves (k) in ihe 

 preceding segments. Neither of these two 

 branches have been delineated by Lyonet in 

 ihe Cossus. The next branch (/) of the trans- 

 verse nerves is given to the trachea' and visceral 

 surface of the great rectus, after wdicd the 

 trunk of tde nerve passes outwards until it 

 arrives at the tuA of tracheal vessels which are 

 situated just behind the spiracle (F). It there 

 divides (m) into two branches, one of which 

 passes on each side of these trachea;. Some 

 filaments from the anterior branch pa-s inwards 

 along the trachea towards the alimentary i anal, 

 while others are distributed to the tran^, r,e 

 lateral muscles, dorsal recti, and lateral mus- 

 cles of the dorsal vessel. The other division 

 of the in rve al-o gives brandies to the trachea; 



Phil. Trans, part ii. 1836. 

 3Q 2 



