t 



PROPORTIONS OF THE TECTORIAL MEMBRANE 57 



do not undergo the growth disturbances undergone by the greater 

 ridge proper, and the fibrous mesenchymal tissue added about 

 them in the formation of the hp aids them in retaining the 

 attachment. This, I am convinced, is, in the pig and probably 

 all mammals, the only attachment of the tectorial membrane 

 after acquiring its functioning form. 



The freeing of the outer zone may not be an absolute neces- 

 sity for the mediation of auditory impulses. In the birds the 

 sensory cells, corresponding to the hair cells of the mammal, are 

 dispersed in a simple epithelial ridge producing both them and 

 the tectorial membrane, and the ridge not receding and disin- 

 tegrating, the membrane remains attached to its parent cells. 

 In the mammal, the otolith membranes of the maculae are homol- 

 ogous to the tectorial membrane, though simpler, and these 

 remain attached in part to their parent epithelium, situated in 

 which are the special sensory cells. Experiments with the 

 model I have tried to construct, imitating the tectorial mem- 

 brane and its environment, suggest that, were the outspanning 

 zone attached, it could be disturbed by vibratory disturbances 

 in the endolymph, though not so readily nor so definitely as when 

 the zone is free. However, in the actual anatomy of the mam- 

 malian cochlea it is evident that this zone is free. Held ('09) 

 cites Pre3^er for the observation that auditory reflexes do not 

 begin in the guinea-pig till one-half hour after birth, and both 

 Held and Rickenbacher found that the outspanning zone of 

 the membrane is freed at about the time of birth. More sugges- 

 tive are the observations of Kreidl and Yamase ('07) upon rats. 

 As is known, the young of these rodents, unlike the guinea-pig, 

 are born in a comparatively fetal stage of development. They 

 found that auditory reflexes do not begin in them till 12 to 14 

 days after birth. Held found in rabbits, which, like the rat, 

 are born in a fetal condition, that the outer zone of the tectorial 

 membrane is not free at 6 days after birth, though it is entirely 

 free in the adult stage. He does not describe cochleae of rab- 

 bits between 6 days after birth and his stages in which the zone 

 is free. 



