52 



HUMAN EMBEYOLOGY. 



constricted into the form of three flat purse-like diverticula which, by the partial 

 obliteration of their cavities, become converted into the three semicircular canals (see 

 Sense Organs). The more ventral part of the dorsal section of the vesicle is divided, 

 by a constriction of its lateral wall, into a dorsal part, the utricle, which remains in 

 connexion with the semicircular canals, and a ventral part, the saccule, which is 

 united to the cochlea by the canalis reuniens. The apex of the constriction which 

 separates the utricle from the saccule passes into the mouth of the ductus endo- 

 lymphaticus, which is thus transformed into the Y-shaped canal which connects 

 the utricle with the saccule. At a later period the closed extremity of the ductus 

 endolymphaticus dilates and forms a small saccule, the saccus endolymphaticus. 

 In the adult the saccus endolymphaticus lies in the posterior fossa of the skull, 



in relation with 

 the posterior 

 surface of the 

 petrous part of 

 the temporal 

 bone and ex- 

 ternal to the 

 dura mater. 



The tympa- 

 num and the 

 auditory tube 

 (O.T. Eustachian) 

 are developed 

 from the first 

 visceral pouch. 



The ventral 

 part of the pouch 

 disappears at an 

 early stage. The 

 dorsal extremity 

 expands and is 

 converted into 

 the cavity of 

 the tympanum, 

 whilst the stalk 

 of connexion 

 with the pharynx 

 is gradually con- 

 stricted off from 

 its lateral to- 

 wards its medial 



end, and is converted into the auditory tube. The constriction commences when 

 the embryo has attained a length of about 20 mm., that is about the beginning of 

 the eighth week, and is completed about the end of that week when the embryo 

 is about 25 mm. long. 



After the auditory tube is defined it grows rapidly in length, and cartilage 

 appears in its walls during the fourth month. 



As the tympanic cavity increases in size the auditory ossicles stapes, incus, and 

 malleus, which are differentiated from the dorsal ends of the cartilages of the first 

 and second branchial arches, are invaginated into it. 



The membrana tympani, which separates the tympanum from the external 

 acoustic meatus, is formed from the separating membrane which intervenes 

 between the first branchial pouch and the first cleft. It consists, therefore, of an 

 external covering of ectoderm, an internal lining of entoderm, and an intervening 

 layer, of fibrous tissue, derived from the mesoderm. 



The external ear is developed from the cavity and the boundaries of the first 

 branchial cleft. The cavity of the cleft is transformed into the cavity of the 

 external acoustic meatus, and on the mandibular and on the hyoid margins of the 



FIG. 69. FIGURES, MODIFIED FROM His, ILLUSTRATING THE FORMATION OF 



THE PINNA. 



1. Tuberculum tragicum = Tragus. 



2. ,, auterius helicis 



3. ,, intermedium helicis 



4. Cauda helicis 



5. Tuberculum anthelicis = Antihelix. 



Helix. 



6. Tuberculum antitragicum = Anti- 



tragus. 



7. Tuberculum lobulare = Lobule. 

 HM. Hyomandibular cleft. 



0V. Otic vesicle. 



