1274 A MANUAL OF ANATOMY 



absent. They therefore establish free communications between the extericr 

 and the fore-part of the alimentary canal, or throat, for the entrance and 

 exit of water. 



The functional branchial arches are those which are post-oral, commencing 

 with the second. The second branchial or gill -arch is not, however, a real 

 branchial arch in the functional sense, but is opercular, giving rise, on either 

 side, to the operculum or gill-cover. The real branchial or gill-arches, properly 

 so-called functionally, are those which succeed to the second, of which the 

 third and fourth are conspicuous. The mucous membrane of the real bran- 

 chial arches is folded into parallel lamellae, which are placed close together, 

 and are freely furnished with capillary bloodvessels, the blood being derived 

 from the branchial -arch arteries. These lamellae constitute the branchiae 

 or gills. 



Aquatic respiration consists in the passage of currents of water, containing 

 oxygen, through the complete gill-clefts into the pharyngeal part of the 

 fore-gut. As the water bathes the branchiae, or gills, its oxygen is taken up 

 into the blood within the branchial capillaries, and the carbon dioxide of 

 the capillary blood is yielded up to the water. Thereafter the water is ex- 

 pelled through the gill-clefts, and is immediately replaced by a fresh current 

 of respiratory water. The branchiae, or gills, of aquatic animals therefore 

 correspond functionally to the lungs of Mammals and Birds, whose respiration 

 is aerial. 



Eustachian Tube. 



The Eustachian tube leads from the tympanum to the naso- 

 pharynx, and is about i^ inches in length. It is directed forwards, 

 inwards, and slightly downwards, and is composed of two parts, 

 osseous and cartilaginous. The osseous or postero-external part 

 is about ^ inch long, and represents the lower or Eustachian com- 

 partment of the canal situated in the angle between the petrous 

 and squamous parts of the temporal bone. The cartilaginous or 

 antero-internal part is about i inch in length, and lies in the groove 

 between the great wing of the sphenoid and the apical portion of 

 the petrous part of the temporal bone. It is at first narrow, but 

 gradually enlarges, so as to resemble a trumpet. The narrowest 

 portion of the whole tube is at the junction of the osseous and 

 cartilaginous parts, this being known as the isthmus, and the widest 

 part is at the pharyngeal orifice. The roof, inner wall, and upper 

 part of the outer wall of the cartilaginous part consist of a triangular 

 plate of cartilage, the margins of which are slightly rolled towards 

 each other. The floor- and most of the outer wall are formed of 

 a dense fibrous membrane. The pharyngeal orifice of the tube is 

 expanded, and is situated on the lateral wall of the naso-pharjmx 

 behind, and external to, the corresponding posterior naris, and on 

 the same level as the posterior end of the inferior turbinate bone. 

 It is somewhat oval, and presents, above and behind, a prominent 

 border, formed by the thick margin of the cartilage, and called the 

 cushion. Posterior to this is the lateral pharyngeal recess or fossa 

 of Rosenmiiller. 



The tube is lined with mucous membrane, which is continuous 

 with that of the tympanum on the one hand, and with that of the 

 naso-pharynx on the other. It is thin in the osseous part of the 

 tube, but in the cartilaginous part it is thick, and contains mucous 



