Archive image from page 85 of Cunningham's Text-book of anatomy (1914). Cunningham's Text-book of anatomy cunninghamstextb00cunn Year: 1914 ( 52 HUMAN EMBRYOLOGY. 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 un
Archive image from page 85 of Cunningham's Text-book of anatomy (1914). Cunningham's Text-book of anatomy cunninghamstextb00cunn Year: 1914 ( 52 HUMAN EMBRYOLOGY. 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 gradually into the mouth of the ductus endolymphaticus, which is thus transformed into a Y-shaped tube con- necting the utricle with the saccule. At a later period the closed extremity of the ductus endolymphaticus, which was originally connected with the surface of the head, becomes dilated and forms the saccus endolymphaticus. This is found, in the adult, in OV . • the posterior fossa of the skull, lying in relation with the posterior surface of the petrous part of the temporal bone. The tympa- num and the auditory tube ( 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 the 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
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