INSECTA. 



947 



by which it is presumed they are thus endowed 

 with voluntary power and sensation. But on 

 reflection it will appear that this ought really to 

 be the case. To enable the insect to make a 

 forcible expiration and close its spiracle, which 

 is evidently an act of volition, the great con- 

 strictor of the spiracle ought to be endowed 

 with voluntary nerves. On the other hand, 

 since, 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, the retractor valvulse ought 

 to be supplied from the 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 the first. It passes diagonally backwards 

 and outwards, and divides into two branches, 

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

 minal muscles, and the second to the triangular 

 and transverse median, while the other (&) 

 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 the moto-sensitive 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 trans- 

 verse nerves 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 his anatomy of 

 Cossus ligniperda. There is a plexus of them 

 in every segment 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 

 corresponding plexus in the Cossus. In our 

 earlier examinations of these nervesf 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 the cords, but from the ganglia. J 

 We have since been satisfied that 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. part ii. p. 544. 



t Op. cit. 1832. 



t Traite Anat. de la Chenille, 1760, also 1762, 

 p. 98 and 204. 



nerves, and which, passing laterally over and 

 very close to the ganglion of the cord, joins its 

 fellow of the opposite side, in the middle line 

 behind it, to form the longitudinal portion of 

 the next 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 con- 

 tain some voluntary motor fibrils. Each plexus 

 is formed of these two filaments, which, 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 other. 

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

 the transverse nerves to unite with the moto- 

 sensitive (f) close to the inner side of the 

 smaller rectus. Near the external 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 (^), 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 (6). 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 (/c) in the 

 preceding segments. Neither of these two 

 branches have been delineated by Lyonet in 

 the Cossus. The next branch (1} of the trans- 

 verse nerves is given to the tracheae and visceral 

 surface of the great rectus, after which the 

 trunk of the nerve passes outwards until it 

 arrives at the tuft of trachea! 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 tracheae. Some 

 filaments from the anterior branch pass inwards 

 along the trachea towards the alimentary canal, 

 while others are distributed to the transverse 

 lateral muscles, dorsal recti, and lateral mus- 

 cles of the dorsal vessel. The other division 

 of the nerve also gives branches to the tracheae 



* Phil. Trans, part ii. 1836. 

 3Q 2 



