286 



RESPIRATION. 



appendage-like process, which reaches the 

 farthest boundary along the roof of the abdo- 

 minal cavity. These appendages may be 

 aptly compared to the abdominal air-cells of 

 birds which communicate with open ex- 

 tremities of the bronchial tubes. The " dia- 

 phragm" in the mammals precludes this 

 interblending of the thoracic and abdominal 

 organs, or the diffusion of air into any of the 

 cavities of the body. 



With reference to the minute structure of 

 the lungs in the saurians, it coincides pre- 

 cisely with the account given of those of the 

 ranidcc. Each septum consists of a central 

 basis or framework of elastic fibrous tissue 

 lined on either side by a reticulate layer of 

 vessels. This plexus is again overspread by 

 a " hyaline pavement epithelium." Rich tracts 

 of ciliary epithelium may be discovered along 

 the margins of cells, the course of vessels, 

 and the lines of condensed structures. The 

 double layer of vessels borne by each septum 

 may be noted as a point of structure dis- 

 tinctive of the reptilian lung. The lungs of 

 the chelonian reptiles are very voluminous. 

 They extend over the whole dorsal part of 

 the trunk as far as the pelvis. They are 

 fixed by the pleura to the ribs, which also 

 separate them from the cavity containing the 

 digestive and generative organs. They are 

 symmetrically developed on the two sides. 

 Through the centre of each lung longitudi- 

 nally an unwa\\ed axis extends from the an- 

 terior to the posterior extremities. This is 

 the main road for the air-currents. From 

 this axis, secondary passages, parietal ly cel- 

 lulated, radiate towards every point of the 

 circumferences of the organ. The ultimate 

 cells are very capacious. They communicate 

 little with each other. Each group has its 

 common outlet, thus resembling a lobule. 

 In the reptilian lung, however, there exist 

 no lobules ; an anatomical particular in which 

 they are distinguished from that of all mam- 

 malia. It is a criterion of lower organisation. 

 The vibratile cilia which line the nasal and 

 buccal passages, the pharynx and oesophagus, 

 the larynx and trachea of all reptiles are 

 most remarkable for tenacity of life in the 

 lungs of the chelonia. In the trachea of the 

 turtle, along certain tracts of the lungs, the 

 motion of cilia may be detected several months 

 after death. The physiological value of the 

 breathing process in any given animal cor- 

 responds, not with the volume of air inspired 

 per any unit of time, but with the measure of 

 the blood-surface exposed to its agency, the 

 rate at which the blood-current moves, the 

 numerical proportion of its red corpuscles,and 

 the frequency of the respiratory movements. 

 The small, but minutely, subdivided lung of 

 the mammal presents a much more extensive 

 surface for the outspreading of the rete mira- 

 bile than the very voluminous, but spacious- 

 chambered lung, of the chelonian. The total 

 volume of air inhaled by the mammal is less 

 than that which the lung of the turtle is 

 capable of containing ; but in the former case 

 it is more minutely distributed and divided ; 



it is more effectually employed ; the contact 

 between it and the blood-web is far more 

 extensive and intimate ; while it acquires a 

 higher temperature than in the latter. In 

 these several particulars, cold differ from 

 warm-blooded animals. 



Respiratory Organs of Fishes. 



The aquatic type, distinctive almost univer- 

 sally of the breathing organs of invertebrate 

 animals, obtains also in the lowest order of the 

 vertebrata. Fishes and the lower amphibia 

 respire on the branchial plan. The difference 

 between a gill and a lung rests more on ap- 

 parent than real and ultimate grounds. In 

 the last anatomical analysis this difference 

 vanishes, and the eye is arrested only by the 

 close structural affinities which reduce the 

 two varieties to an essential unity of type. In 

 both, the blood is exposed to the agency of the 

 aerating element by means of reticulated ves- 

 sels, furnished with distinct parietes, and pre- 

 senting a diameter little in excess of that of 

 the corpuscles of the blood ; so that these 

 latter must travel through the true respiratory 

 capillary in a single series. This fact denotes 

 the extreme measure to which the subdivision 

 of the blood-stream is carried. It is a funda- 

 mental requirement of the breathing organ, 

 that all structures interposed between the 

 blood and the surrounding element should be 

 reduced to the utmost degree of attenuation. 

 Accordingly, it is found that the epithelium 

 overlying the rete mirabile consists of a single 

 layer of attenuated scales, perfectly destitute 

 of those contained parts which give bulk and 

 density. In no instance whatever within the 

 limits of the vertebrtita (excepting, as stated 

 already, the branchire of the amphibia) are 

 the true respiratory capillaries covered by a 

 ciliated epithelium. This rule applies also 

 to the branchiae even of the higher inverte- 

 brata, such as the Crustacea and cephalopoda. 

 The gill of the fish differs from that of the 

 crustacean in the extreme minuteness with 

 which the blood current is subdivided, and in 

 the existence of specially parieted vessels ; 

 conditions which denote an ' intensified 

 measure in the function of breathing in the 

 instance of the vertebrate animal. In the 

 blood of the vertebrata the floating cells are 

 infinitely more numerous, relatively to the 

 bulk of the fluid, than in that of the inverte- 

 brata ; a fact more expressive than the 

 former of the greater activity of the respira- 

 tory process in the vertebrate than in the in- 

 vertebrate animal. 



It is an axiom in physics, that no gas is 

 capable of passing through an organic septum 

 without first assuming the fluid form. This 

 axiom destroys the apparent difference be- 

 tween a gill and a lung. In contact with the 

 gill the aerating medium is already fluid: in the 

 case of the lung, it takes this condition only in 

 the act of passing through the partition dividing 

 the blood from the external medium. Between 

 the gill of the fish and the true lung ofthe ver- 

 tebrate animal there is discernible, however, 

 this differential character, that in the former 



