PROPORTIONS OF THE TECTORIAL MEMBRANE 31 



might be advanced that vibration of both the tectorial mem- 

 brane and the membranous spiral lamina in case of very strong 

 sound stimuli would be an economical arrangment in the struc- 

 ture of the cochlea. By such, the tectorial membrane, whose 

 motion alone actually stimulates the hair cells, may in these 

 cases be kept within working distance of .the protruding hairs, 

 or by such may be obviated injuriously forcible impingement of 

 the tectorial membrane upon the hairs, which might result from 

 extraordinarily great excursions produced in it by the strong 

 stimuli, were the lamina to remain undisturbed. In the more 

 ordinary and less strong sound stimuli, amplitudes and vibra- 

 tion frequencies must occur which are incapable of throwing 

 the lamina into vibration at all but which may affect the tec- 

 torial membrane, its excursions decreasing, with the strength 

 of the stimuli, to the functional limit of the auditory apparatus. 

 I hope to discuss more fully the probable action of the tec- 

 torial membrane with a description of experiments with the 

 model referred to above. 



STRUCTURE OF THE TECTORIAL MEMBRANE 



In the study of the tectorial membrane of the adult hog, 

 I have found little reason to modify the conclusions drawn in my 

 previous paper as to structure from its study in cochleae of fetal 

 pigs. It consists of fibrils or filaments of very varying lengths 

 imbedded in a gelatin-hke, probably keratin ous, matrix. The 

 directions in which its fibrils are arranged result from the gradual 

 changes in position and the increase and decrease in number of 

 the individual cells which produce its fibrils. The greater epithe- 

 lial ridge, which produces the membrane, grows thicker, wider 

 and longer for a considerable period after it has begun produc- 

 tion, and then recedes in thickness and width and finally dis- 

 appears as a producing structure. The axial side of the ridge 

 ceases to grow and its cells cease to produce the membrane 

 earlier than the outer side of the ridge. Thus the fibrils of the 

 axial side of the membrane are the shorter and merely curve, 

 from the basal side axisward, due to the growth in width of the 



