404 JOHN H. STOKES 



now available in this laboratory, it was finally decided, however, 

 that the scope of the present study might well be limited to the 

 Weigert preparations. 



Finally, it should be understood that the drawings of the models 

 herewith presented represent an effort to reduce the apparatus 

 under consideration to something of its simplest terms. Accord- 

 ingly disputed points and minute details have been largely 

 omitted, and attention called to such omissions in the text. The 

 corpus ponto-bulbare, and the anterior commissural tract in rela- 

 tion with the nucleus lemniscus lateralis are so distinctive in 

 position and relations that they are represented in spite of the 

 fact that they may still be considered as disputed points. 



COCHLEAR APPARATUS 



In the general topography of the opossum hindbrain, the 

 relatively enormous size and prominence of the primitive sensory 

 and motor groups forming the central apparatus of the cranial 

 nerves, is as striking as is the enormous development of the pons 

 in the human brain. Such sensory groups as the fifth and the 

 eighth stand out with startling distinctness, and the identity of 

 the main units in the structure is seldom in doubt for a moment. 

 Even on the external surface of the brain stem the bulge of the 

 spinal fifth, and the corpus trapezoideum and its offshoot in the 

 form of the lateral lemniscus, are easily made out. The posterior 

 colliculus and the medial geniculate body are even more striking 

 external landmarks. The opossum brain at first glance therefore, 

 impresses one as especially well adapted to the mapping out of 

 large primitive sensory groups by reconstruction. 



The general structure of the cochlear apparatus is essentially 

 the same in the opossum as in the more familiar types, and its 

 main divisions may be similarly outlined, somewhat as follows. 

 The chain of auditory conduction begins with the cochlear nerve 

 proper, consisting of the axones of bipolar cells in the spiral gan- 

 glion. This cochlear nerve is accompanied by the vestibular 

 nerve, the two together forming the eighth cranial or acoustic 

 nerve of ordinary nomenclature. The cochlear nerve terminates 



