The Ear 301 



The formation of the external ear, or pinna, has not been so 

 well studied in domestic animals as in man. The conchal carti- 

 lage arises from the mandibular and hj-oid arches, which bound 

 on either side the hyomandibular cleft. As shown in Fig. 36 in 

 the human ear, the concha consists of a series of tubercles with 

 deep fissures extending between them. We have no data to 

 show that the early stages of development of the concha in our 

 domestic animals are precisely parallel, but they are presumably 



Fig. 36. The left ear of a human embryo, lettered by Professor 

 His, Br. 2, and estimated as thirty-five days old. From ]\Iar- 

 shall, after His. X 20. 



I, tuberculum tragicum. 2, tuberculum anterius helicis. 3, 

 tuberculum intermedium helicis. 3 and 3c, cauda helicis. 4, tuber- 

 culum anthelicis. 5, tuberculum anti.ragicum. 6, tubercu- 

 lum lobulare. 



essentially so. B}^ observing Fig. 36, it will be seen that be- 

 tween I and 2 there is a deep fissure which, in the ear of the 

 horse, is apparentl}- marked by an important ridge inside the 

 concha. It is interesting, in connection with this fissure, to ob- 

 serve that foals are frequently born with a deep invagination of 

 the epiblast at this point, causing a long, narrow fistula which 

 extends downwards from about the middle of the internal border 

 of the concha to near its base, and from which there exudes a 

 viscid mticus. In other cases, this invagination extends more 

 deeply and penetrates the squamous temporal bone and, in the 

 •development of the osseous tissues, the invagination is inter- 

 rupted and a closed sac formed at the distal end, in which, 

 ordinarily, one or more teeth are formed (ear teeth) which 

 resemble more or less closely the molars of the horse and may 

 grow to almost any size, projecting far above the external sur- 

 face or growing inward, causing an inward bulging of the skull 

 into the cranial cavity. (See Figs. 37 A and B.) Various authors 



