CONTAINED PERIOTIC TISSUE-SPACES IN THE HUMAN EMBRYO. 43 



In dealing with the cistern and also with the scala? one should not consider 

 them as insignificant accessories that merely fill in the waste intervals between the 

 membranous labyrinth and the surrounding cartilage. From studying their devel- 

 opment it becomes apparent that they have a morphological individuality in many 

 respects as definite as that of the ossicles themselves. They make their appear- 

 ance at a definite time and at definite places, they spread in a definite manner, and 

 eventually they attain a form and structure that are adapted to a definite function. 

 This becomes more and more evident as we examine older stages. 



The form and relations of the scalre in fetuses between 12 and 13 weeks old are 

 shown in figures 28 and 29. These figures show median and lateral views of a wax- 

 plate reconstruction of the membranous labyrinth and the surrounding periotic 

 spaces in a human fetus 85 mm. crown-rump length (Carnegie Collection, No. 

 1400-30). Attention has already been directed to these figures in the description 

 previously given of the cistern. The scala vestibuli can be seen in figure 28. Above, 

 it opens freely into the cistern and extends downward along the apical side of the 

 duct as a single main space, possessing a rather uniform diameter. It extends 

 along the first two turns of the duct, gradually tapering off and showing a less 

 mature character in its distal portions. Along the second turn of the duct the 

 spaces are incompletely fused and the contour becomes correspondingly irregular. 

 As a rule the peripheral margin of the scala is less mature and more irregular than 

 the central margin. The scala vestibuli does not connect with the scala tympani 

 at any point as yet. The two are separated in the first place by the cochlear duct 

 and then more centrally by a framework of connective tissue in which are the 

 radiating bundles of the cochlear nerve with the nodes of ganglion cells, that form 

 the spinal ganglion. These latter structures are not shown in the model; they 

 occupy, however, the V-shaped groove seen between the two scalse. 



The scala tympani, as can be seen in figure 29, extends downward on the basal 

 side of the cochlear duct along its first two turns. This corresponds to about the 

 same linear dimension as that of the scala vestibuli. In its proximal portion it 

 shows a greater area in cross-section than the latter, but further toward the apical 

 region it is of about the same size and in some places it is even smaller. The periph- 

 eral margin of the scala tympani is distinctly more irregular than the central 

 margin. The irregularity is due to spaces along this margin that are actively 

 coalescing with the main space, but in which the fusion is not yet complete. The 

 irregularity of this margin is thus an indication of the direction of the expansion of 

 the scala. As the diameter of the whole cochlear mass increases, it is evident that the 

 main growth of the scala must radiate outward in a peripheral direction. This is 

 accomplished by the continual assimilation of new reticular spaces along this margin. 

 At the proximal end of the scala tympani can be seen an oval depression which 

 corresponds to the fenestra cochleae (rotunda) and with which it stands in intimate 

 relation. 



In fetuses about 16 weeks old the form and relations of the scalae have nearly 

 attained the adult conditions, and this represents the oldest stage studied in con- 

 nection with the present paper. The conditions found at this time are shown 



