STRUCTURE OF THE VERTEBRATES 



145 



is the tympanum or ear drum, and the opening into the pharynx- 

 is the Eustachian tube. Recall the position of the hyomandibu- 

 lar, its proximal end in contact with the otic capsule. Its posi- 

 tion in the amphibia is unchanged, except for its inclusion 

 within the middle ear. Therefore the hyomandibular of the fish 

 becomes the columella of the amphibia, and is the ear ossicle 

 which assists in transmitting sound waves from the tympanum 

 to the inner ear which contains the organs of equilibrium and 

 the rudimentary hearing apparatus. 



Brat 

 Cavity 



Pharynx — Spiracle j ^Pharynx 



PharynxA; 



'Eustachian' 

 tube 



A- Dogfish 



B. Froff 



C. Mammal 



Fig. 82. Cross sections of the Pharynx (Diagrammatic). The spiracle (A) 



develops into the middle ear and Eustachian tube (B), closed by the ear 



drum. The Mammal (C) shows the deep position of the ear drum and 



the elongated Eustachian tube. 



In the reptiles and birds the same conditions exist in regard 

 to the three replacement bones under discussion: (1) the articu- 

 lar, (2) the quadrate, and (3) the columella. In the mammal- 

 like reptiles the replacement bones of the jaws become smaller, 

 and an accessory point of articulation develops between the der- 

 mal dentary and squamosal. 



The embryo of the mammal demonstrates the final evolution 

 of the three jaw cartilages. The hyomandibular cartilage re- 

 mains as an ossicle of the middle ear, and is called the stapes 

 in the mammal. The minute end of the pterygo-quadrate car- 

 tilage ossifies in close contact with the hyomandibular and is 

 also included within the middle ear cavity as a second ossicle, 

 the incus. The third mammalian ossicle, the malleus is a deriva- 

 tive of the ossified ]Meckel's cartilage, and is homologous with 

 the articular bone of the amphibia and reptiles. 



