1^ ON THE CIRCULATION OP THE BLOOD, 



gene the animal soon expires. We have already observed that some 

 amphibials appear to possess only a single heart, and even that of a very 

 simple structure. 



In fishcF the heart is single, or consists only of two compartments instead 

 of fo. f, and hence the circulation is single also The gills in this class 

 answer the intention of lungs, and the blood is sent to them for this pur- 

 pose from the heart, in order to be deprived of its excess of carbone, and 

 supplied with its deficiency of oxygene. It is not returned to the heart, as 

 in the case of the superior animals, but is immediately distributed over the 

 body by an aorta or large artery issuing from the organ of the gills. The 

 oxygene, in these animals is separated from the water instead of from the 

 air : and for this purpose the water usually passes through the mouth be- 

 fore it reaches the gills : yet in the ray-tribe there is a conducting aperture 

 on each side of the head, through which the water travels instead of through 

 the mouth. In the lamprey it is received by seven apertures opening on 

 each side of the head into bags, which perform the office of gills, and 

 passes out by the same orifices, and not as has been supposed, by a differ- 

 ent opening said to constitute its nostril. 



In the common leech there are sixteen of these orifices on each side of 

 the belly, which answer the same purpose. In the sea-mouse (aphrodita 

 aculeata) " the water passes through the lateral openings betw^een the 

 feet into the cavity under the muscles of the back."^ 



The siren possesses a singular construction, and exhibits both gills and 

 lungs ;t thus uniting the class of fishes with that of amphibials. Linn^us 

 did not know how to* arrange this curious animal, and shortly before his 

 death formed a new order of amphibials, which he called meantes, for the 

 purpose of receiving it. It ranks usually in the class of fishes. 



The only air-vessels of the winged insects have a resemblance to the 

 apertures of the lamprey, and are called stigmata. In most instances 

 these are placed on each side of the body ; and each is regarded as a dis- 

 tinct trachea, conducting the air, as M. Cuvier elegantly expresses it, iir 

 search of the blood, as the blood has no means of traveUing in search of 

 the air.| They are of various shapes and number, and are sometimes 

 round, sometimes oval, but more generally elongated like a button-hole. 

 In the grasshopper they are twenty-four, disposed in four distinct rows. 



The membranous tube that runs along the back of insects is called by 

 Cuvier the dorsal vessel. It discovers an alternate dilatation and contrac- 

 tion ; and is supposed by many naturalists to be a heart, or to answer the 

 purpose of a heart. Cuvier regards it as a mere vestige of a heart with- 

 out contractions from its own exertion, and without ramifications of any 

 kind : the contractions being chiefly produced by the action of the mus- 

 cles running along the back and sides, as also by the nerves and tracheae, 

 or stigmata. Scorpions and spiders have a.proper heart : and as the term 

 insects is now confined by M. Cuvier and M. Marcel de Serres to those 

 that have ordy this dorsal vessel, or imperfect heart, the two former ge- 

 nera are struck out of the list of insects as given by Linneus.§ 



This organ diflfers very considerably in its structure and degree of sim- 



* Sir E. Home, Phil. Trans. 1815, p. 260. 



t Home's Life of Hunter, prefixed to Hunter's Treatise on the Blood, Inflammation, &c. 

 p. xli. 



t En un mot, le sang ne pouvant aller chercher l air, c'est Pair qui va chercher le sang. 

 Lecons d'Anat. Comp. i. 23. Sect. 2. Art 6. 



§ See M. Marcel de Serres' statement, Tilloch's Journal, vol. xliv.p. 14a. ; and eepecially 

 Thomson'H Annals of Phil. No. XXIU. p. 347, 348. 350. 354. 



