280 



RESPIRATION. 



their vascular system {figs. 227, 228.) Like 

 those of the ranidae they are clothed in a 

 vibratile epidermis, numerously starred by 

 pigmental cells, in common with the rest of 

 the body. For some time before the deca- 

 dence of these organs in the larvae of the triton 

 they cease to exhibit the phenomenon of ciliary 

 vibration. The vibratile epidermis undergoes 

 a change by which the ciliated cell becomes 

 succedeed by the simple. This event foretells 

 the approaching extinction of the parts. In 

 their earliest condition the branchiae of the 

 newt discover only four minute simple cylindri- 

 cal filaments. Each "rows in length and thick- 



O ~ 



ness, and throws out from the inferior surface 

 a double row of pectinated processes. These 

 are more complexly constructed than the pri- 

 mitive filaments. They carry not only an 

 afferent and efferent trunk, but an elaborate 

 plexus of capillary vessels. The pigment cells 

 are limited in their distribution to the larger 

 lobes, and to the line of the larger vessels. 

 The epidermis of the secondary processes of 

 the branchiae is reduced to extreme tenuity. 

 Through it the eye readily tracks the move- 

 ments of the individual blood corpuscles on 

 the branchial capillaries. These elliptical bodies 

 move like a boat, their long axes coinciding 

 with that of the channel in which they are 

 travelling. Sometimes several proceed abreast. 

 The diameter of the vessels of the temporary 

 branchiae is greater than those of the lungs. 

 In general terms it can be confidently stated 

 that the quantity of blood circulating in the 

 temporary branchiae of the amphibia, at the 

 period of their maximum development, is far 

 less in relation to the amount contained in the 

 whole body than that which the lungs, when 

 fully formed, are capable of carrying. This 

 inferior amount of blood is physiologically ex- 

 pressive of an inferior functional power in the 

 case of the temporary organs. Their respira- 

 tory function is really only supplemental to 

 that of the whole body. The whole cutaneous 

 surface, as in the Nacmatoid annelids, is richly 

 ciliated. It is organised like the branchiae. 

 On these parts, however, the epidermal layer 

 is not so attenuated as that with which the 

 branchias are invested. On these latter there 

 is, however, a very perceptible epidermal co- 

 vering. Its scales exhibit the ordinary hexa- 

 gonal figure. 



This demonstration, which dispels all doubt, 

 establishes the physiological principle, that 

 the presence of epithelium is compatible with 

 the respiratory office of the part which it 

 clothes. This laiv prevails throughout the 

 class of fishes ; it has also been reduced to 

 actual fact by the author throughout the whole 

 sub-kingdom of the invertebrata. But it must 

 not be forgotten that its office on the breath- 

 ing organs is almost exclusively mechanical. 

 In no known example among vertebrated 

 animals does the epithelial investment of 

 a respiratory surface develop itself into any 

 of the forms of a secreting organ. No 

 " follicles " are, at any time or under any 

 circumstances, discovered on these localities, 



in this class of animals, but in the inver- 

 tebrata follicular glandules are constant on 

 the surfaces of the respiratory organs. The 

 constituent scales are therefore function- 

 ally passive. Nuclei and a granular proto- 

 plasm would find no purpose to subserve if 

 they were present in a highly developed form. 

 Thus is exemplified the law of " demand" 

 and "supply:" disuse entails attenuation on 

 all living structures. Either the gases by 

 which the epidermis of respiratory localities 

 is traversed suppress the glandular office of 

 the scales, or these latter, from the first, re- 

 ceive a special organisation. The scales of 

 the branchial epithelium contain nothing but 

 a pellucid fluid. This fluid condenses, fluidi- 

 fies, the respiratory gases in transitu. This is 

 the office of the refined covering under study. 

 The "principle" that the epithelium of the 

 breathing organs is required by the physical 

 conditions of its office to be reduced to the 

 state of the utmost thinness receives new 

 proofs from the study of the internal branchiae. 



From all that is known it is probable that 

 in minute structure the branchiae of the per- 

 enni-branchiates conform to the plan of the 

 temporary organs just described. The ge- 

 neral arrangement of the primary branchial 

 vessels and the structure of the heart are 

 identical. 



The internal temporary Branchiae of the 

 Amphibia. The process by which these or- 

 gans are withdrawn into the interior of the 

 branchial chamber is not simply that of short- 

 ening. It is the labour of a new organisation. 

 The internal gills of the tadpole differ in type 



Fig. 229. 



The internal bronchia of the Tadpole of the Frog. 



b, e, are the primary trunks supported by the 

 cartilaginous arch c, which give oft" the looped 

 processes a; d, is one of the vascular loops viewed 

 transparently, and showing the arrangement of 

 vessels in them, and the epithelium by which they 

 are covered. 



of structure from the external. The ultimate 

 vessels of the latter are differently looped. 



