THE MEDULLA OBLONGATA AND CEREBELLUM 



151 



it follows that their primary centers are correspondingly highly 

 developed and distinct. The medulla oblongata, in fact, is 

 divided into four longitudinal columns related respectively to 

 the great primary groups of functional systems. In fishes, 

 where the amount of correlation tissue is less than in man, these 

 four primary columns appear as well-defined ridges in the wall 

 of the fourth ventricle. 



An enlarged view of the medulla oblongata of the sturgeon, 

 which is very similar to that of the dogfish, is seen in Fig. 68, 

 which also illustrates the arrangements of the primary sensory 

 and motor centers in cross-section at several different levels. 



Somatic sensory column 



Visceral sensory column 



Visceral motor column 



Somatic motor column 



Fig. 67. Diagrammatic transverse section through the human spinal 

 cord. Compare Figs. 56 to 59 and note the relatively greater size of the 

 dorsal gray columns and dorsal funiculi in man than in the fish (Fig. 66). 

 This is correlated with the greater importance in man of the ascending 

 connections between the cord and the brain (see p. 129). 



Figure 69 shows a cross-section through the medulla oblongata 

 in the region of the vagus nerve in another fish, the sea-robin. 

 In all of these cases the four principal functional systems (see 

 pp. 76 and 79-94) are arranged in longitudinal columns from 

 the dorsal to the ventral surface in the order: somatic sensory, 

 visceral sensory, visceral motor, and somatic motor centers, as 

 indicated diagrammatically on the left side of Fig. 69. The 

 arrangement of the peripheral nerve-fibers of these systems is 

 indicated on the right side. Figure 70 illustrates a cross-section 

 through the corresponding region of the medulla oblongata in an 

 early human embryo, where the same general arrangement of 

 the sensori-motor centers is evident. 



