310 MANUAL OF HISTOLOGY. 



to appear. In the posterior region is a large tract (see Fig. 135) 

 containing- scattered groups of many small cells evidently con- 

 nected with the arciform fibres. This is probably a part of 

 the lower origin of the pneumogastric. A little in front and 

 external to this is a small group of larger nerve-cells which 

 help to form the lower sensory origin of the fifth nerve. Still 

 farther forward in the lateral region is a large collection of 

 multipolar nerve-cells. Although this group is traversed in 

 many directions by fibres, single and in bundles, still it seems 

 to give rise to fibres which run back and upward, evidently to 

 curve upon themselves and join the peripheral fibres of the 

 spinal accessory root. (See Figs. 135 and 137.) 



Farther forward still there is a collection of small cells 

 arranged in a wavy line (see Fig. 135), the commencement of the 

 olivary nucleus. Through this the roots of the hypoglossus 

 all pass. Some seem to be lost in it, others appear to arise 

 from it, but this is probably due to the arrangement of roots 

 often seen to curve into the nucleus and then out again. As 

 this is the first appearance of the olivary body, it will be well 

 here to describe it. 



THE OLIVARY BODY. 



The olivary nuclei are situated in the medulla, under the 

 oval projections on its anterior surface called the olivary bodies. 

 The nucleus consists of a strip of gray matter arranged in gen- 

 eral like a piece of fluting folded on itself, so as to form almost 

 an ellipse. From the concavities of the fold on either side pro- 

 ceed bundles of fibres, the external ones joining the formatio 

 reticularis, the internal ones passing into the raphe. Their 

 connection with the hypoglossal roots is probably not 'im- 

 portant. The intimate structure of the olivary fold is that of a 

 dense gray matrix holding numerous small polyhedral cells 

 having delicate protoplasmic processes. 



Let us now go a trifle higher (see Fig. 137), and observe that 

 in sections the central canal, which has all along been elongat- 

 ing and receding backward, now opens into the apex of the 

 fourth ventricle. There is now, therefore, quite a deep notch in 

 the posterior part of the section, covered with the same cylin- 

 drical epithelium which lined the central canal. On each side, 



