54 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



The Eustachian Tube. — The Eustachian tube is derived from 

 the first inner cleft recess of the pharynx (Fig. 39) and retains 

 through life the ciliated epithelial lining of the primitive pharynx. 

 It is 1^ inches long. Its inner J| is bounded behind by a 

 triangular plate of cartilage, which is attached at its inner or 

 pharyngeal end to the internal pterygoid plate, a derivative, with 

 the incus, of the pterygo-palatine bar (Fig. 10 C). The cartilage 

 is developed in the walls of the 1st visceral cleft in the 4th 

 month of foetal life. The tympanic plate grows inwards and 

 forms the floor of its outer third (Fig. 40), while the periotic 

 capsule (petro-mastoid) which is developed above and behind the 

 1st cleft, grows over and forms the roof of its outer third. The 

 part of the petro-mastoid which grows over it, is the tegmen 

 tympani ; it also forms the roof of the tympanum and antrum of 

 the mastoid. It forms the roof of all the cavities derived from 

 the inner recess of the first cleft (Fig. 40). The anterior edge 

 of the tegmen tympani appears in the Glaserian fissure. The 

 tensor tympani and tensor palati are developed on the mandibular 

 side of the first cleft and are supplied from the nerve of the 

 mandibular process through the otic ganglion. 



tegmen tympani 



squam. 



int. carotid /^IPIP^^^ 



Sftympan.,^-^^ drum 



meatus 



tympanic plate 

 Eustach. cartil. 



Fig. 40.— Showing the Cavities derived from the Inner Recess of the First Cleft. 



The Tympanum. — As may be seen from Fig. 39, the tympanum 

 can scarcely be said to exist at the sixth week of foetal life. The 

 inner cleft recess ends in the jelly-like tissue containing the 

 cartilaginous bases of the malleus and incus. It is directed out- 

 wards and backwards between the periotic capsule to its posterior 



