18 tnBaM.ATOKV SVHTEM. 



tubw by the oMcr analoinbtii ami i»h)!«lt»l«»«l»W, \» accordant with knuwn fncto in the 

 comwny of tiw lowiTanimaUjainunK witi«h It is common for nn orRan to i»crfi>nii n dou!»Ic 

 rbnrtion. 



TIm- f'XNl ill tlir nliiiiiiitary raiinl of Iumi t<( i-« movid along iiy a JK-ri.*t«ltir motion : th«* 

 canal, tln-n-f'>rf, I* •iii|>|>iir<l witii a nm.sciilnr ii|>|>iirnlUA. Imle-*-*!, from lli« plunomtita 

 cxhil>it«il. wf ml^iit Infer tliiit the- n]>|NirntUH has u ron^tnution uml pnrt!« such us Itavc 

 pincr in iiun<lriiiK'»l<i ; that tin* routs of th«- iiiimi'Mtary cnmtl iin- thn-**, n nmcoa«, a ccllu- 

 Ur, and a muscular cojit ; thr first In-InK tin- lnt<Tnal on*-, tin- mii.scular tiic outside, and 

 the cclliilur In the midtllc. Tin* mucous coal h thin, oftrti trans|Minnt, and difficult to 

 detect. The muscular c«mt is |>i-rri-ctly d«veloi>tHl : its fibres arv lioth lonRiludina] and 

 circular ; aiid where the constrictions iH-culiar to the tulie exist, there it is more jH-rfcctly 

 develo|K'd, or in Rreater l'..rce. Thes<> mu-eb-s si-rve |>iirtly us *|>hincten», wid also to kct-p 

 uj> a coMtiniiul movement. 



CiHcii.ArioN-. Ius«'cls are furnished uilli a rireuiatory aj>i>aratus : it consists of a dorsal 

 vessel, whicli, as its name intlicates, is a tuln- running Uneatli the jlorr>ul face of the ex- 

 ternal coverins;. This is the central or^an, and is e«juivalent to tl>e heart in the higher 

 orders of animals ; and the circulation is continued from the dorsd vc-ssel, in cIiunneN 

 excavate<l in the tissues. The structure of the dorsal vessel i>ermits the bl<>ud to flow from 

 the |)osterior to the anterior i>urt of the nnimul, by means of a series of valves oi>eniiig 

 forwartls. The bbxMl is transmitted tu the thorax and head, the wings and legs, and returns 

 through certain channels denominated ru/ij, which ojk« into the dorsal at diflt-rent fH>iut5 

 through the valves ; but the larger iH.rtiou of the returned fluid collects in the i«.slerior 

 chamlnT, ami |'ass<s the whole length of the vesM-1. The vess«l has lK-<-n, and still is, re- 

 garded by soMie i>hysiob>gists as anab'gous to the heart : others, however, cuiiiider it as 

 morv analogous to the aorta of quadru|K-ds. 



It il<H»s not seem to Ik* a matter of much imiH)rtance whether we regard the so-called 

 dopial vessel n heart, an aorta, <»r an organ compounded of both, the i>osterii>r segment 

 taking more «-s|K-cinlly the form and function of :in aortic vessel. Its visible contractions, 

 ■a s«><'n throiich llie tnmsparent covering of many ins«'cts, furnish ostensibly valid reas<iQ9 

 for rrRarding if a heart. 



The rtuJiN which circulate in the va.scular system of insects are usually white, carrjing 

 along distinct cor{>uscules having forms like those which niv found in the higher and more 

 perfect animals. 



In conni-ction with the foregoing facts, It should also be slated that the circulation in 

 Insects Is carrbnl on in v«»s.<t4'ls hoving close pniximity with those of another kind, namely, 

 the air-vessels, or trarhm. The two chi.sses are easily distinguished from each other by the 

 structure of the latter ; f>>r in ..rder that air may tniverse the insect system, it is necessary 



