THE SENSE-ORGANS 497 



there is no lobule and little or no recurving to the free 

 edge. This latter peculiarity, which starts at the upper part of 

 the base, is distinctively human, but extends over a varying dis- 

 tance in different individuals, and is often hardly begun in the 

 neiv-born infant. A rudimentary point, tuberculum auriculi 

 \_Darwini~\, is often retained at the free edge, and is brought 

 over by the rolling process so as to point forward instead of 

 backivard, its primary position. This is. occasionally a con- 

 spicuous feature, and in all cases its place can be determined 

 by feeling, being indicated by a thicker, harder area on the 

 outer rim a little below the top of the curve. A lobule is 

 usually present, but is rudimentary or absent in certain races. 

 - Concerning the origin of the cartilaginous elements of the 

 external ear, it becomes evident from the condition found in 

 monotremes that it is largely or wholly derived from the upper 

 end of the hyoid arch, which curves about the tympanic mem- 

 brane and forms a tubular meatus together with a rudimentary 

 pinna. This leaves unaccounted for a series of protuberances 

 in the integument surrounding the opening of the meatus, 

 which are seen to form in the human embryo and fuse to- 

 gether to build up the external portion (pinna). These pro- 

 tuberances are considered by some to be elements furnished 

 by the first four visceral arches, i.e., mandibular, hyoid and 

 the first two branchial, but this is rendered very improbable 

 by the innervation of the pinna, which is wholly from the 

 Facialis. It may thus prove to be a modification of devel- 

 opment, and portions which were originally hyoid elements may 

 here appear in this form. In the Sauropsida the outer tym- 

 panic membrane is frequently depressed a little below the sur- 

 face and provided with small protuberances or flaps which as- 

 sist in its protection, but these are evidently incidental adapta- 

 tions and can have nothing to do with the external ear of mam- 

 mals. 



The developmental history of the eye, as given in the pre- 

 vious chapter, shows that this sense-organ, that is, its essential 

 part, the retina, differs radically from all the others in being 

 originally a portion of the brain surface, the cells of which have 



