RESPIRATION. 



285 



one on either side of a fibrous partition. A 

 plane of vessels disposed in such a manner 

 can only receive the influence of the aerating 

 element on one side. This fact constitutes 

 a real anatomical distinction between the 

 lung of a reptile and that of a mammal. In 

 this latter case the partitions of the cells are 

 composed only of a, single stratum, both sides 

 of which are "exposed to the air. By this 

 simple mechanical provision the amount of 

 the respiratory agency is everywhere doubled. 

 In the structure of the reptilian lung the 

 elastic fibre forms a predominant element. 

 It is a substitute for ribs and other accessory 

 apparatus of breathing. The lungs of the 

 frog, relatively to the cubic capacity of their 

 interior, present a much more extensive 

 active surface than those of the Salaman- 

 dridae. Thus the purpose of "the cells" is 

 fulfilled, of multiplying the operative surface. 

 The "septa" project from the sides into the 

 interior of the organ. In this respect they 

 may be likened to the gills of fishes ; for, 

 like the latter, " they penetrate the surround- 

 ing medium." The lungs of ophidian rep- 

 tiles are generally composed of true unsym- 

 metrical, long cylindrical or fusiform sacs, 

 extending from the pharynx far into the 

 cavity of the abdomen, above the other 

 viscera, and surrounded with the serous 

 lining of that cavity. They are capable of con- 

 taining a considerable quantity of air, which, 

 when driven out with force, produces the 

 "hiss" peculiar to the serpent. In some ge- 

 nera, as the coluber, typhlops and vipera, the 

 lung of one side only is developed ; in others, 

 as the boa and python, the two lungs are sym- 

 metrically or equally developed. The lungs, 

 in these families, communicate, by means 

 of a long and narrow trachea, surrounded 

 by incomplete cartilaginous "rings," with 

 the back part of the tongue. In all ophidia, 

 the lungs display internally, but only on the 

 anterior and upper parts, an elaborate system 

 of alveoli or cells, more like secondary lungs 

 than air-cells. The posterior two thirds 

 of the internal superficies are almost plane, 

 or devoid of" cells," like the lung of the newt. 

 The alveoli, traced from before backwards, 

 become shallower and shallower, until at 

 length they disappear. It thus appears that 

 the serpent may store up in its lung a con- 

 siderable volume of air which, slowly passing 

 out over the vascular air-cells, prevents the 

 carbonic acid, the effete product of the pro- 

 cess, from contaminating the whole contents 

 of the organ. Each " alveolus," separately 

 examined by vertical section, is found to 

 communicate by a single opening with the 

 general chamber of the lung. Traced inwards, 

 it divides and subdivides into secondary and 

 tertiary tiers of "alveoli," each cell being 

 isolated by dissepiments of which the struc- 

 ture is identical with those of the frog's lung 

 already described. Each cell is a separate 

 cavity. It does not communicate with those 

 adjacent by openings in the septa. These 

 septa are utilised in the outspreading of the 

 vascular rete. Each septum, aa in the frog, 



carries two layers of capillary blood-vessels, 

 separated from each other and supported by 

 an intermediate stratum of elastic tissue. 

 In the mechanism of breathing, this tissue 

 enacts an important office. Over the interior 

 of the ophidian lung, like the batrachian, the 

 ciliated epithelium is limitedly distributed. 

 The true capillary areas which chiefly co- 

 incide with the flat sides and bottoms of the 

 cells are clothed only with 'hyaline epithelium.' 

 Everywhere throughout the interior of the 

 lung, along the courses of the larger vessels, 

 the borders of cells, or along lines of thick- 

 ened tissue, the phenomenon of ciliary vibra- 

 tion may be readily detected. It is thus evi- 

 dent that the office of cilia is mechanical, if 

 not to cause determinate currents in the air, 

 at least in the halitus and fluid which, by 

 accumulating, may obstruct the respiratory 

 function of the capillary areas. 



In the boa and python the length of the 

 left lung is generally less by a third or half 

 than that of the opposite side ; but in coluber, 

 crotalus and others, it is much smaller and 

 quite rudimentary, appearing as little more 

 than an obliterated appendage. The genera 

 CcBcilia and Amphi&bcEna have the left pulmo- 

 nary organ developed, and the right short- 

 ened : this arrangement probably varies ac- 

 cording to the species. Vipers and other 

 serpents possess only a single lung, which 

 on that account is very long. The lungs 

 of the saurian reptiles conform in character 

 to those of the ranidce and salamandridcs. 

 They are elongated sacs, cellulated internally. 

 They extend far back along the roof of the 

 abdominal cavity. Like those of the ophi- 

 dians, they are divisible into a cellulated and 

 smooth or non-cellulated portion. The 

 former is limited to the upper and anterior 

 half of the organ ; the latter to the inferior 

 wall and posterior half. There may be a 

 mechanical reason in this peculiar distribu- 

 tion of parts. The cells in the lungs of the 

 saurians exhibit none of the regularity so 

 characteristic of those of the ophidians. They 

 are larger and more irregular. The partitions 

 of the cells are more slender and more de- 

 licately membranous. 



The whole interior of the lungs in the 

 higher saurians is multiplied into cells. An 

 axis without definite walls, like a trachea, 

 runs from one end of the organ to the other, 

 as is the case in the lung of the turtle. 

 From either side of this axis, large orifices 

 lead to the more subdivided portions, or 

 secondary and tertiary air-chambers. On the 

 contrary, each lung in Scincus ojficinalis forms 

 a single continuous cavity ; but the entire 

 surface of the parietes is cellulated by small 

 projecting reticulate septa. The internal 

 dorsal and anterior half of the lung of the 

 chameleon is, as usual in the sauria y minutely 

 cellulated. Further back the cells become 

 larger, and the septa smaller, until at the 

 posterior part the walls consist only of plane 

 membrane both less vascular and less can- 

 cellated than the anterior. The coscal ex- 

 tremity of the organ is drawn out into an 



