THORAX. 



1019 



pitl vibration of hair-like organs cilia. ( Vid. 

 Art. CILIA.) 



Of the Second Species of Respiration. In- 

 sects. The breathing apparatus in insects 

 generally reaches a high degree of develop- 

 ment. Sometimes respiratory tubes or tra- 

 cheae penetrate every part, in the form of mi- 

 nute ramifying vessels conveying the included 

 air to all the organs. The moving power or 

 means of the renewal of the air ift these tubes 

 is at present little known. Some insects, al- 

 though they live in water (as, for instance, the 

 water beetles, and also water spiders), retain 

 a bubble of air around them ; and, according 

 to Nitzch*, they renew the air in the irtrachae 

 by alternately elevating and depressing the an- 

 tennae. Beetles, before flying, seem to inflate 

 themselves with air, so as to unfold their 

 wings, which, like other parts of their body, 

 are supplied with air tubes. In this case 

 an inspirative effort must be made by some 

 cavity. Under certain circumstances, bees 

 emit a voice, a shrill sound, which is indepen- 

 dent of the motion of the wings, and which 

 appears to be connected with the existence of 

 a current of air through the respiratory tubes 

 or tracheae; at least, such has been observed 

 when the animal has been irritated and im- 

 mersed in water, the surface of which, where 

 it was in contact with the orifice of the stig- 

 mata at the root of the wing, evidently vibrated 

 at the moment the sound was produced. \ 



In the orthoptera particularly, there are 

 distinct respiratory movements, alternate dila- 

 tations and contractions of the abdomen ; in 

 fact, respiratory motions are more distinctly 

 perceptible in this division than in any other in- 

 sects. In the locusta vcrrucivora particularly, it 

 is easy to distinguish how the abdominal rings, 

 which have smaller abdominal scuta between 

 them inferiorly, are alternately elevated and 

 depressed exactly like ribs. If we smear the 

 great thoracic stigmata with oil, we find that 

 numerous bubbles of air escape from it during 

 these motions. 



The organs of respiratory motion, by means 

 of which the supply of air is renewed, present 

 many points of uncertainty. On the one 

 hand, where large stigmata are placed opposite 

 to each other, and connected by tracheae, it is 

 easy to see that alternate opening and shutting 

 of their valves may produce a current capable 

 of renewing the supply of air. It is conceiv- 

 able also, how, in the orthoptera, lepidoptera, 

 and others, the expansion and contraction of 

 the body, and the elasticity of the air-sacs 

 contained in it, may cause the ingress of air. 

 It is less obvious, however, how the same 

 effect is produced in caterpillars and the larvae 

 of beetles, where a current of air cannot very 

 easily arise from the opposite position of the 

 stigmata, on account of the minute ramifica- 

 tions of the tracheae ; and consequently we 

 must look for some peculiar mechanism, pro- 

 bably cilia, unless we are disposed to admit 



* On the Respiration of the Hydrophilse, in Reil's 

 Archiv. B. x. S. 440. 



f Hunter, Philosophical Transactions. 1792. 

 p. 182. 



the stagnation of the air in its vessels. Hence 

 it has been conjectured that the dilatation and 

 contraction of the dorsal vessel contributes to 

 this purpose. This, however, appears to be 

 scarcely possible ; and it might be asked on 

 the contrary, if the vermicular motion of the 

 body itself, the sliding of its segments upon 

 each other, are not the means of keeping up 

 the constant ingress and egress of air.* 



In the lowest of the molluscous class, the 

 external tunic with which they are covered 

 is generally so elastic, that it is capable of 

 dilating by its own properties, when it has 

 been greatly contracted by the muscular coat 

 that is within ; and in forcible expirations, 

 Dr. Grant has observed these animals to 

 contract their muscular coat, and to retract 

 the exterior covering, so as to propel, with 

 considerable impetuosity, and to a distance, 

 the water that fills their respiratory cavity. 

 The elasticity of the tunic tends to over- 

 come the resistance of the muscular coat, 

 and to expand, to a certain extent, the respi- 

 ratory cavity. Without, therefore, the existence 

 of elastic ligaments, such as we find in conchi- 

 fera, there is a partial means of enlarging the 

 respiratory cavity given to these tunicated 

 animals. This, however, is only in occa- 

 sional, forced, respiration ; constant and al- 

 ternate contraction and expansion of the ex- 

 terior tunic is not met with in any known tu- 

 nicated, nor in a conchiferous animal. *f- The 

 streams that enter the respiratory, and pass 

 out of the anal aperture, are smooth, regular, 

 and incessant, and are produced by ciliary 

 movement. 



Of the Third Species of Respiration. Fishes. 

 It may be said that the thorax of fishes 

 usually presents four elastic cartilaginous 

 arches, which approximate and separate, open 

 or close the gills, at the same time increasing 

 or diminishing the capacity of the so-formed 

 thorax. These ribs, or branchial arches, sup- 

 port the gills, which are covered by a great 

 flap (operculum) on each side of the base of 

 the skull. 



The respiratory current enters at the 

 mouth, passes through the fissures on each 

 side of the fauces, and escapes through the 

 branchial openings, placed laterally, covered 

 by the moveable operculum. This stream is 

 uniformly in one direction, from before, 

 backwards. It might be asked, why does not 

 the water rush in by the branchial opening 

 when the mouth " threatens" a vacuum ? It 

 will be observed that the margin of the oper- 

 culum, or great lateral flap, is edged with a 

 delicate membrane, which acts as a valve, this, 

 by the pressure of the water, is forced close 

 round the lateral openings : thus, the water, 

 upon the expansion of the jaws, is prevented 

 entering behind, and consequently rushes in 

 towards the gills by the mouth ; the jaws now 

 close, the operculum immediately opens by the 



* Reimarcus, Ueber das Athmen. in Eeil's Archiv. 

 B. xi. S. 2. ; and Xitzsch, Comment, de Respirat, p. 

 39. et seq. 



f Grant's Lecture " On the Respiratory Organs 

 of Invertebrata." Lancet, 18334, vol. i. p. 904. 



