RESPIRATION. 



331 



nldcs), the internal integumentary membrane 

 is materially aided in the performance of this 

 function by the aquiferous canals in the 

 former, and by the peritoneal cavity in the 

 latter ; indeed, the peritoneal cavity may be 

 considered the special organ of respiration in 

 the Echinidae. 



The respiratory membrane, in most cases, 

 presents one of three forms, which have re- 

 ceived the names of gills or branchice, of 

 trachefs, and of lungs. In the gills or 

 branchiae the tegumentary membrane is pro- 

 longed outwards, in the shape of laminae, 

 tufts, or branches; and this arrangement is 

 found in aquatic animals. This form of the 

 respiratory membrane is not, however, uni- 

 versal in those aquatic animals possessing 

 special organs of respiration. In the Ho- 

 lothuridae, one of the tribes of the Echinoder- 

 mata, the chief respiratory organ consists of 

 two aquiferous tubes, of an arborescent form, 

 that open upon the surface of the cloaca.* 



In the Ascidians, among the Mollusca, the 

 chief respiratory organ is a large cavity, re- 

 garded by some as a dilatation of the ceso- 

 phagus ; and in certain of the aquatic Gastero- 

 poda it consists of a sac, with lamellae on its 

 inner surface, opening upon the external sur- 

 face of the body. The small cavities placed 

 along the sides of the body of the leech, and 

 opening externally, are also believed to be 

 respiratory organs. 



The arrangement of the respiratory mem- 

 brane, termed trachea, is present in the Arti- 

 culata, among the Myriapods, insects, spiders, 

 and also, with a few exceptions, among the 

 larvae of insects living in the atmosphere ; 

 and is observed in greatest perfection in the 

 adult insects. It consists of a prolongation 

 of the external membrane into the interior of 

 the body, in the form of tubes, often ex- 

 tensively subdivided and ramified, kept open 

 by fibres rolled round their walls in a spiral 

 manner, and commencing at the external sur- 

 face of the body by orifices termed stigmata. 

 In certain of the larva? of insects, this ar- 

 rangement of the respiratory membrane is 

 modified to adapt it for aquatic respiration. 

 In the larvae of the Ephemera, these tracheae, 

 instead of terminating in stigmata, are prolonged 

 outwards into a foliaceous expansion of the 

 external integument, where they are subdi- 

 vided and ramified, and terminate in shut ex- 

 tremities. A constant interchange between 

 the air in the tracheae of these larvae, and 

 the atmospheric air dissolved in the water, 

 will go on through the membranes interposed 

 between them.f In the larvae of the Libella 

 the tracheae are distributed in a similar man- 

 ner in a membrane placed within the anus, 

 and the animals draw in and expel the water 

 with considerable force from that cavity, so 



* This form of the respiratory apparatus has been 

 termed coelobranchiate by Straus-Durckheim, being 

 derived from *O?A?, hollow ; and /3g*)a<*, gill. 



f This modification of the tracheal respiratory 

 organ has been designated tracho-branchiate by 

 Straus- Durckheim. 



that these respiratory movements act at the 

 same time in causing locomotion. 



The arrangement of the respiratory mem- 

 brane called lungs consists of the prolonga- 

 tion of the tegumentary membrane inwards in 

 the form of sacs, and is destined for aerial 

 respiration. In some of the terrestrial gas- 

 teropodous Mollusca, the lung is formed by a 

 single, large, and simple sac, opening by an 

 orifice on the right side of the body. In the 

 Arachnida the lungs are composed of two or 

 more separate cavities, lamellated on their in- 

 terior, opening on the external surface of the 

 body, and are analogous to the branchial cavity 

 in some of the aquatic Gasteropoda. In all 

 the air-breathing Vertebrata, the respiratory 

 membrane is formed by a prolongation of the 

 internal tegumentary or mucous membrane 

 from the upper part of the digestive tube, and 

 this also holds in the aquatic Vertebrata, or 

 the fishes. When the expanded respiratory 

 membrane is placed at some distance from 

 that portion of the mucous membrane of the 

 digestive tube with which it is continuous, as 

 is especially the case in the Mammalia and 

 birds, this mucous membrane is prolonged to 

 the part where its expansion occurs, in the 

 form of a tube, strengthened on the outer sur- 

 face by elastic textures to enable it to with- 

 stand the atmospheric pressure. Along this 

 tube (trachea), and its branches (bronchi 

 and bronchial tubes), the air passes to and 

 from the proper respiratory membrane on the 

 inner surface of the lungs. In the water 

 newt the lungs consist of a pair of elongated 

 sacs, without any internal laminae or folds. 

 In the frog these membranous sacs present 

 ridges on their inner surface, especially at the 

 upper part; and in the lungs of the turtle and 

 crocodile these ridges increase in number and 

 in size, and form partitions dividing the interior 

 of the lungs into numerous cells communi- 

 cating with each other. 



In birds the bronchial tubes on entering 

 the lungs have numerous parietal cells on 

 their inner surface ; and this extension of the 

 respiratory surface is still further increased by 

 some of the tracheae opening into membra- 

 nous bags, often presenting a cellular appear- 

 ance, and communicating with the interior of 

 certain of the bones. In the lungs of man 

 and the other Mammalia, the bronchial tubes 

 divide and subdivide into minute branches, 

 each of which ends in a cluster of terminal 

 cells, forming one of the small lobes into 

 which the lung may be divided. By this 

 arrangement an immense extent of respiratory 

 surface is packed up in a small space.' 



* 



* Hales (Statical Essays, vol. i. p. 242. 1769) 

 estimates the inner surface of the whole lungs in 

 the calf at 289 square feet, equal to 19 times the 

 surface of a man's body ; and Lieberkuhn calculates 

 (as quoted by Schultz in his System der Circulation, 

 p. 288) that the whole surface of the air-tubes and 

 air-cells in the human species amounts to 1400 

 square feet. Monro, on the other hand (Essays of 

 Monro Secundus, p. 59. Edinburgh, 1840) calcu- 

 lates that the inner surface of the human lungs is 

 only equal to 440 square feet, or nearly thirty times 



