506 EMBRYOLOGY. 



the higher forms. Tbeir "function is to transmit vibrations to the 

 labyrinth, and consequently they are together called the conducting 

 apparatus. From their position they are also known as middle and 

 outer ear. The former consists in Mammals, where it attains its 

 highest development (diagram, fig. 284), of the tympanic cavity (Ct), 

 the Eustachian tube (Tb), and the three auditory ossicles (SAp) ; the 

 latter, of the tympanic membrane (Mt), the external meatus (Mae)> 

 and the external ear or auricle (M). The statement just made, that 

 these parts are wanting in Fishes, is to be taken cum grano salis ; it 

 is as a sound-conducting apparatus only that they are wanting, for 

 they are present even in the case of Fishes, but only as structures 

 of a different function and in a more simple condition. For the 

 various accessory apparatus of the organ of hearing are developed 

 out of the first visceral cleft and certain parts which are located in 

 its periphery. 



Here also it will be well to acquaint ourselves with the original 

 the initial condition, for which the Selachians may serve as an 

 example. 



In them the greater part of the first visceral cleft, which is 

 situated between the mandibular and hyoid arches and between the 

 nervus trigeminus and n. acustico-facialis, disappears ; at the side 

 of the throat it becomes closed, remaining open only at the origin, or 

 base, of the two visceral arches. It then has the form of a short 

 canal, which possesses a small round opening at its inner and 

 another at its outer end, and which passes in very close proximity to 

 the labyrinth-region of the skull, in which the organ of hearing is 

 located. The canal, here called the spiracle, has no longer anything 

 to do with respiration, since the branchial leaflets on its wall have 

 undergone degeneration. Owing to its position in the immediate 

 vicinity of the labyrinth, it presents, even in the Selachians, the best 

 course for the propagation of the sound-waves to the inner ear, and 

 this is the chief ground for its entering wholly into the service of 

 the organ of hearing in the remaining Vertebrates, and for its being 

 developed in a more serviceable manner for this particular function. 



The structures in the higher Vertebrates corresponding to the 

 spiracle of the Selachians are (fig. 284) the tympanic cavity (Ct), 

 the Eustachian tube (Tb), and the external meatus (Mae). They 

 likewise are developed out of the upper part of the first visceral 

 cleft. Although it has recently been asserted by certain investi- 

 gators (URBANTSGHITSCH) that they have nothing to do with the 

 first visceral cleft, but are established independently by the evagina- 



