RESPIRATION. 



279 



those only in which the external gills are per- 

 sistent throughout the whole term of adult 

 life. Whether temporary, as in the caduci- 

 branchiate, or persistent, as in the perenni- 

 branchiate genera, the branchial organs of 

 amphibia are supported by no skeletal frame- 

 work analogous to that which sustains the 

 soft parts of the breathing apparatus of fishes. 

 They are, essentially, only " productions," un- 

 der a modified form, of cutaneous structures. 

 Contemplated only as a mechanical contri- 

 vance, whether provisional or permanent, up- 

 on which devolves the most important func- 

 tion in the animal economy, it demands a 

 more minute investigation than it has hitherto 

 received at the hands of anatomists. 



The cartilaginous arches erected on the 

 hyoid bone do not entirely disappear until the 

 internal gills have ceased to be distinguish- 

 able. The circulating system of the ciecidual 

 branchiae consists, in its earliest stage, of a 

 simple artery and vein ; that is, a loop of one 

 vessel. As the larva grows these two vessels 

 become separated by an intermediate system 

 of capillaries. In the latter phase they offer 

 no remote analogy to the vascular apparatus 

 of the branchiae of fishes. The cardiac centres 

 are composed only of a right auricle and one 

 undivided ventricle ; the left or pulmonary 

 auricle remains unevolved until the organic 

 necessities attendant on growth create a ne- 

 cessity in the system for the exercise of the 

 pulmonary functions. The left auricle is then 

 superadded, and the chamber of the ventricle 

 is partially divided by a median partition, and 

 the embryonic organism reaches the maximum 

 limit of development. The pulseless ventral 

 artery, the resultant of the united afferent 

 vessels of the branchiae, undergoes oblitera- 

 tion through disuse. These general observa- 

 tions form no irrelevant introduction to a 

 more special examination of the branchial 

 organs. 



Temporary external Gills. The larval 

 branchiae of the frog and toad are less endur- 

 ing and less complex than those of the sala- 

 mandridae. From the earliest almost to the 

 latest moment of their existence they are 

 furnished with a ciliated epidermis. The gills 

 are not specially ciliated The whole cutaneous 

 surface in the larva of the frog and toad is 

 similarly endowed. The cilia are in active 

 play for some time before the larva emerges 

 out of the egg : an admirable instance of 

 foresight in the provisions of nature. The 

 covering of the external gills of the ranidae is 

 strictly cutaneous. In this situation, as every- 

 where else, the epidermis betrays its real nature 

 by the presence of pigmental cells. It is little 

 less dense than the ordinary covering of the 

 body. Nor does the vascularity of these tem- 

 porary branchiae much exceed that of the rest 

 of the cutaneous surface. These facts pro- 

 claim their provisional character. At first 

 they consist of a single minute lobe. This 

 increases into two and then into several. They 

 are cylindrical, not flattened, processes. They 

 bear a single vessel returning upon itself. In 

 this particular of ultimate structure they dif- 



fer from the branchiae of the salamandridae. 

 In these a capillary net-work is constructed 

 between the artery and vein. This greater 



Fig. 228. 



One of the gills of the Newt viewed transparency. 



{Original.) 



a indicates the right auricle, which with r, the 

 ventricle, constitutes the heart of the true fish ; a' 

 shows the left or pulmonary auricle, which, being 

 superadded to the two former parts, raises the car- 

 diac organ to the reptilian standard, marked by the 

 presence of two auricles and one incompletely par- 

 titioned ventricle ; b, b', denote the circuit of the 

 branchial system in conformity with the pisciform 

 type. This system of vessels being obliterated 

 during the metamorphosis of the larva, the pul- 

 monary vessels (c c') enlarge, the rudimentary lungs 

 at the same time expand, the associated auricle 

 grows in muscularity and dimensions, and the fish 

 rises to the grade of an air-breathing reptile, d, is 

 an enlarged view of the gill of the larva of the newt 

 soon after its escape from the ovum. Secondary 

 processes {d, d, d, d) are extended backwards, which 

 materially multiply the surface. The whole gill is 

 clothed with ciliated epithelium, the cells of which 

 lose their cilia, and become non-vibratile for some 

 time before the cessation of the branchial breathing, 

 and the oblong-cilia-bearing cells (e) are transmuted 

 into epidermal scales ( f) entirely destitute of cilia. 



elaboration coincides with their longer dura- 

 tion. What is ephemeral in purpose is tem- 

 porarily formed. This is nature's workman- 

 ship. They consist literally of small pro- 

 longations of 'the skin, which is everywhere, 

 as here, ciliated. At the moment of their 

 fullest development, the larval branchiae of 

 the frog consist of four filamentary lobes. 

 These are sessile upon the body or stem of 

 the branchiae ; they are somewhat granular 

 on the surface, and slightly irregular in form. 

 There is also frequently a short additional 

 branch at the base of the posterior one. In 

 these interesting organs the movement of the 

 blood is readily demonstrated. It is a beauti- 

 ful spectacle. It advances in a single current 

 along one side and returns along the other. 

 No sooner have these exquisite organs at- 

 tained their greatest development than they 

 bec;in to diminish in size. They become ob- 

 tuse, and are gradually so reduced as to be 

 withdrawn within the branchial cavity, and 

 concealed by a little operculum of the integu- 

 ment. The nature of this change of structure, 

 which attends the transition of the branchiaa 

 from the external to the internal condition, 

 has never yet been defined by anatomists. It 

 will be immediately described. 



The external Gills of the Salamandridce ex- 

 ceed the former in size, in the number of the 

 appended lobules and in the complexity of 



