144 



THE NERVOUS SYSTEM. 



Fig. 99. 



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

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

 horns. 



Small nerve cells are irre- 

 gularly distributed through- 

 out the whole of the grey 

 matter and cannot be 

 grouped; 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 



are : 



i. The nucleus centralis 

 (upper inner group, Reiss- 

 ner), nucleus medullae oblon- 



Transverse section through hinder end of Me.dulla 



dblongata (magnified 30-80)!, from Stieda. 

 6 Inferior commissure. 

 / Dorsal horns. 

 fl Ventral horns. 

 h Fourth ventricle. 

 i Nucleus centralis. 

 k Isolated mass of grey matter in which longitudinal f/atde, Stieda (Fig. 99 i), IS 



fibres of the pneumogastric nerve co\irse. n t> 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'C^o-cvc^S mm. long by 0*020 mm. 

 broad. 



2. The auditory nucleus (nucleus acusticus, Reissner, Stieda), 

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

 fourth ventricle opposite 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 nerve radiate from their superficial Origin in all 



1 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. 



