THE EAR. 433 



the circumference to the centre of the membrane ; others come 

 directly from the long ciliary arteries and others again from 

 the arteries of the crystalline capsule. These several vessels 

 are found principally on its posterior face. Its veins have not 

 been observed. s 



This membrane first shows itself about the third month of 

 foetal existence, and is most perfect at the seventh ; from the 

 latter period it begins to decline, by disappearing from the cen- 

 tre to the circumference. At the ninth month it consists only 

 in a few loose flocculent masses adhering to the pupil. M. J. 

 Cloquet has ascertained that its vessels do not participate in its 

 destruction, but that the arches which they form are retracted 

 to the margin of the pupil, and there form the lesser arterial 

 circle of the iris. From the observations of Drs. Jacob and 

 Tiedemann, it 1 appears that traces of the membrana pupillaris 

 exist for ten or fifteen days after birth. The latter, in one case, 

 injected its vessels in a foetus at full term.* 1 



CHAPTER III. 



OF THE EAR. 



THE ear, the organ of hearing is placed principally within 

 the petrous portion of the temporal bone, and consists in the 

 External Ear, or Auricle, the Tympanum, and the Labyrinth. 



SECT. I OF THE EXTERNAL EAR. 



The position of this portion of the organ is familiar to every 

 one. It is useful in collecting the rays of sound, and in con- 

 veying them to the more internal parts. It is formed by the 

 structure, exterior to the petrous bone, called, in common lan- 

 guage, the Ear; and by a bony canal which leads internally to 

 the tympanum. The basis of the first portion is cartilaginous, 



* Am. Med. Jour. vol. i. p. 192. 

 VOL. II. 38 



