OF THE RESriRATION. 63 



part of the liver, which projects as a point. This vena cava 

 runs a short distance, and enters as a principal part in the 

 formation of that venous sac described above, which com- 

 municates with the right auricle, and in which several other 

 veins terminate : a species of valvules are observed at the 

 mouth of each of these veins.* 



The heart of Ophidians has constantly on its upper sur- 

 face a conglomerate gland, more or less in size, and sur- 

 rounded by several other glands differing much, according 

 to the species, in form and number. This first gland has 

 been regarded by several naturalists as analogous to the 

 thymus ; by others, and I am of this number, it has been 

 compared to the thyroid, t 



OF THE RESPIRATION". 



To obtain an idea of the respiration of Ophidians, we 

 must examine with attention a serpent in the state of repose. 

 We then observe that the trunk contracts and dilates alter- 

 nately by the play of the ribs, and that the movements are 

 slowly repeated at regular intervals ; but we observe also, 

 that the nostrils remain closed during this operation, and 

 only open at considerable intervals, during which the body 

 often has contracted thirty times. It results from these 

 remarks, that the lungs of Ophidians, besides their ordinary 

 function, have to act as a reservoir for atmospheric air : 

 this reservoir, filled by a single inspiration, contains a suf- 

 ficient quantity of air, in order that the oxygenation of the 

 blood can take place by the contractions of the lungs, with- 

 out the necessity of a new inspiration, which only follows 

 the expulsion of the air contained in the lungs, from which 

 the oxygen has been totally absorbed. 



The structure of the lungs of Ophidians clearly proves 

 w^hat we advance ; it is chiefly remarkable for two circum- 

 stances, ^rsi, because the lung usually terminates in a blad- 



* The description of the organs of circulation has been chiefly taken 

 from the Boa constrictor. For further details consult the labours of 

 CuviER, Leqons cVAnat. Comp., vol. iv. ; Sciilemm, in Tiedemaxn Zcit- 

 schrift, vol. ii., p. 1, pi. 7 ; Retzils' Schwed Verhandl., 1830, and the Is^is, 

 1832, p. 524 ; Meckel, System, vol. v. p. 218. 



t Consult the excellent memoir of Haugstedt, entitled Thymi De- 

 scription p. 152. 



