RESPIRATION. 



283 



attention is invited. The conclusions here- 

 after to be drawn will be found opposed to 



Fig. 232. 



A small portion of the lung of the Newt laid open and 

 examined by transmitted light, under a high power, 

 such that only the surface (internal or mucous) is 

 in focus. ( Origin al. ) 



a, a leading branch of the pulmonary artery, 

 giving off at very regular intervals which break at 

 once into the true capillaries e, c, c. ; d, d, d, denote 

 the parenchymous islets which fill up the meshes of 

 the capillary plexuses. (They are the true pul- 

 monary parenchyma.} g, marks the abrupt line which 

 abruptly limits the distribution of the ciliated epi- 

 thelium, which follows the larger vessels in tracts ; 

 d, c, c, c, coinciding with the true respiratory or 

 capillary areas of the lung, are seen to be destitute 

 of ciliated epithelium. 



the views of Mr. Rainey. This excellent 

 observer* has affirmed the principle that, on 

 the true breathing portions, or capillary seg- 

 ments of the lungs, there literally exists no 

 epithelial lining of any description whatever, 

 the vessels being as literally naked. To this 

 " principle," deliberately enunciated and sup- 

 ported by elaborate "proof" by an acute and 

 truthful observer, many anatomists have yielded 

 implicit assent. First, it is here objected that 

 such a "principle" violates directly all the 

 lessons of analogy. Analogy ! Is not demon- 

 stration better than analogy ? In the science 

 of organised beings, the connected reasoning 

 founded on analogy cannot be despised. The 

 closest scrutiny in individual instances may 

 miss the truth. The manifold illusions of the 

 microscope may readily mislead. Analogy 

 supposes a mass of cumulative evidence. The 

 general law neutralises particular errors. In 

 no instance whatever, either in the vertebrate 

 or invertebrate kingdom, has it been proved, 

 in the course of the present extended inquiry, 

 that the vessels of a real breathing organ can 

 exist under a perfectly " naked form." What 

 is true of invertebrate animals as an organic 

 law cannot be untrue of the vertebrate. The 



* Med. Chir. Trans. 1848. 



gills of fishes are furnished with a very 

 marked epithelial covering. The temporary 

 branchiae of the amphibia are clothed with 

 epidermis. The air-bladder is provided with 

 an epithelial lining, the cells of which admit 

 of ready and conclusive demonstration. Why 

 should it not exist in the case of the true 

 pulmonary structures ? No reason can be 

 imagined ; but the fact that it does not has 

 been affirmed by Mr. Rainey. Mr. Rainey's 

 observations were instituted upon injected 

 preparations. This is the source of the error 

 into which he has fallen. If the lung of the 

 newt be carefully, but quickly, laid open, co- 

 vered, but not pressed, with a thin slip of glass, 

 and examined under the microscope, it will 

 be found that the vibratile cilia have a limited 

 distribution. Under the favourable oppor- 

 tunities afforded by such a preparation it is 

 perfectly easy to follow with the eye the 

 continuation of the epithelial cells (c, c, fig. 

 232.) beyond the limits of the ciliary areas 

 into the true capillary or active breathing 

 segments. The ciliated portions of the epi- 

 thelium (over the vessels b, b) exhibit a 

 flocculeni character, precisely as shown in the 

 preparations of Mr. Rainey ; while the areas 

 immediately adjacent appear smooth or naked. 

 But under the use of a higher power and a 

 steady gaze the polygonal outlines of the epi- 

 thelial scales can be distinctly discerned most 

 readily between the islets of parenchyma (W, d). 

 If this covering consist of "basement mem- 

 brane," then basement membrane is composed 

 of scaliform parts ; but it is not. It is a true 

 and real and unbroken continuation of the 

 tracheal and bronchial mucous membrane. 

 It is only the ciliary appendages to the cells 

 that cease at a certain limit ; the cells them- 

 selves continue to invest the whole super- 

 ficies of the lungs. It is full of interest also 

 to note that the epithelial scales which cover 

 the capillary areas of the lung of the newt 

 (parts which coincide with air-cells of the 

 mammalian lung) lose not only the external 

 appendages (cilia), but also their internal 

 parts (nucleus and granules). This succes- 

 sive reduction leaves nothing but a hyaline 

 involucrum enclosing a pellucid fluid. This 

 is the real structure, supported indeed by a 

 hypothetical basement membrane, by which 

 the capillary areas of the pulmonary organs 

 are invested. It finds a parallel in the trans- 

 parent scales which cover the cornea. 



In these ribless amphibia the operation of 

 breathing resolves itself into an act of" swal- 

 lowing" air. The glottidean chink is em- 

 braced by two minute semilunar pieces of 

 cartilages and furnished with muscles for 

 opening and closing the orifice. In the pa- 

 rietes of the lungs no trace of muscular fibres 

 can be discovered ; but elastic fibres are 

 present everywhere among the vessels. It is 

 by the agency of this elastic tissue, aided by 

 the abdominal parietes, that the act of expira- 

 tion is performed. The exterior of the lung 

 is lined by peritoneum, the scales of which are 

 much attenuated compared with those of 

 other parts of the same membrane, as those 



