COMPARATIVE ANATOMY. 



Bbides these cells, a considerable por- 

 tion of the skeleton is formed into recep- 

 tacles for air in most birds, (for there 

 are indeed exceptions and considerable 

 variations in the different genera and spe- 

 cies.) This structure is particularly mark- 

 ed in the larger cylindrical bones, as the 

 scapula, clavicle, and femur. It is also 

 found in most of the broad and multangu- 

 lar bones of the trunk, as the sternum, 

 ossa innominata, dorsal vertebrae, &c. 

 All these are destitute of marrow in the 

 adult bird, at least in their middle ; so 

 that the cylindrical bones form large 

 tubes, which are only interrupted to- 

 wards the extremities by a sort of trans- 

 verse bony fibres : the broad bones are 

 filled with a reticulated bony texture, 

 the cells of which are empty. They have 

 considerable apertures, (most easily 

 shewn in those extremities of the cylin- 

 drical bones which are turned towards 

 the sternum) communicating with the 

 lungs by small air-cells ; which facts may 

 be shewn by various experiments on liv- 

 ing and dead birds. 



These receptacles of air probably serve 

 the purpose of lightening the body of 

 the bird in order to facilitate its motions. 

 This effect is produced in most birds to 

 assist their flight ; in some aquatic spe- 

 cies for the purpose of swimming ; in 

 the ostrich and some others, for running-. 

 Hence we find the largest and most nu- 

 merous bony cells in birds which have 

 the highest and most rapid flight, as the 

 eagle, &c. And hence also the bones of 

 the bird which has just left the egg are 

 filled with a bloody marrow, which is 

 absorbed soon after birth, entirely in 

 some, in others, particularly among the 

 aquatic species, at least for the greater 

 part. 



Besides the uses which have been al- 

 ready pointed out, these receptacles of 

 air diminish the necessity of breathing 

 frequently, in the rapid and long con- 

 tinued motions of several birds, and in 

 the great vocal exertions of the singing 

 birds. 



The lungs of amphibia are distinguish- 

 ed from those of warm-blooded animals, 

 both by a great superiority in point of 

 size, as well as by a greater' looseness of 

 texture, arising from the great size of 

 their air-vessels. In frogs, lizards, and 

 serpents, the lungs consist of a cavity, 

 whose sides are cellular. The poste- 

 rior part of the organ either forms a 

 mere membranous bag, or else the 

 cells are larger there than elsewhere. 

 In the turtle the vesicles are verv 



large, but the texture is uniform through- 

 out. 



In the tadpole, and the young of such 

 lizards as bring forth in water, there are 

 two organs, which somewhat resemble 

 the gills of a fish (appendices fimbriatx, 

 Swammerdam.) These serve for the 

 purposes of respiration while the animal 

 lives in the water. They are connected 

 to the sides of the neck, and hang loose 

 from the animal ; they are not permanent, 

 but are gradually withdrawn into the 

 chest, (within a few days, in the reptiles 

 of this country, )where their remains may 

 still be perceived for some time neap tq 

 the true lungs. Instead of the branchial 

 opening, by which fishes again discharge 

 the water which they have taken in at 

 the mouth, some, tadpoles have for this 

 purpose a canal on the left side of the 

 head, near the eye ; which must be dis- 

 tinguished from the small tube on the 

 lower lip, by which they attach them- 

 selves to aquatic plants. 



Instead of lungs, fishes are furnished 

 with gills or branchiae ; which are placed 

 behind the head, on both sides, and have 

 a moveable gill cover (operculum bran- 

 chiale,) which is wanting in the order of 

 pisces chondropterygii only. By means 

 of these organs, which are connected 

 with the throat, the animal receives its 

 oxyen from the air contained in the wa- 

 ter ; as those animals which breathe de- 

 rive it immediately from the atmosphere. 

 They afterwards discharge the water 

 through the branchial openings (apertu- 

 rae branchiales?) and therefore they are 

 distinguished from animals of the three 

 preceding classes by this circumstance ; 

 viz. that they do not respire by the same 

 way that they inspire. 



We have already shewn, in speaking of 

 the organs of circulation, how the gills 

 receive the venous blood by means of 

 the branchial artery, and how this 

 blood is sent into the aorta after its 

 conversion into the arterial state. The 

 distribution of these vessels on the 

 folds and divisions of the gills consti- 

 tutes one of the most delicate and mi- 

 nute pieces of structure in the animal 

 economy. 



Each of the gills consists in most 

 fishes of four divisions, resting on the 

 same number of arched portions of 

 bone or cartilage, connected to the 

 os hyoides. Generally there is only a 

 single opening for the discharge of the 

 water; but in many cases, particularly 

 among the cartilaginous fishes, there are 

 several. 



