144 



THE NERVOUS SYSTEM. 



longitudinal fissure deepens until the two meet in the fourth ven- 

 tricle; beyond this point one cannot speak of dorsal and ventral 

 horns. 



Fig. 99. Small nerve cells are irre- 



gularly distributed through- 

 out the whole of the grey 

 matter and cannot be 

 grouj^ed; the larger cells, 

 on the other hand, are 

 arranged in distinct groups 

 which have special relations 

 with the nerves arising 

 from the part. Occasion- 

 ally these groups are not 

 so isolated as usual ; in 

 this case processes of one 

 group can be traced into 

 another group (Reissner). 

 Of these groups the chief 



Transverse section through liinder end of Medulla are * 



oblongata (mygnified 30-80)1, from Stieda. 

 b Inferior commissure. I. The miclcUS Centralis 



g vZfJ'ZZ. (^PPer inner group, Reiss- 



h Fourtii ventricle. ngr) nucletis mechillae oblon- 



i Nucleus centralis. ' c\ • •\ /tt -x • 



ft Isolated mass of grey matter in which longitudinal gutae, Stlcda (Fig. 99 l), IS 

 fibres of the pneumogastrio nerve course. r» ni; i j. 



^ ^ a group or cells round to- 



wards the hinder end of the medulla oblongata, on either side of and 

 below the central canal ; the group can be traced under the floor of 

 the fourth ventricle to about its middle. The cells are rounded or 

 spindle-shaped, the processes directed upwards, downwards, or out- 

 wards ; their average size is o-040-o*048 mm. long by 0*020 mm. 

 broad. 



2. The auditory nucleus {iincleus acusticus, Reissner, Stieda), 

 (Fig. 100 n) is a large group of cells found in the wall of the 

 fourth ventricle oj)posite the point of origin of the auditory nerve. 

 The cells are rounded, pear-shaped, or of spindle form, and inter- 

 spersed between the nerve fibres ; these cells have an average 

 length of 0-040 mm., and are about half as broad. The fibres 

 of the auditory nei-ve radiate from their superficial origin in all 



' In these diagrams, from Ludwig Stieda's Studien iiber das centrale Nervensystem 

 der Wirbelthiere, the outline of the diagram is magnified thirty times, while the 

 details are magnified eighty times. 



