MEDULLA OBLONGATA OF AMBLYSTOMA 351 



p. 285) in a recent description of the cranial nerve components of 

 Siren notes a separate general cutaneous root and ganglion of the 

 facial nerve. Coghill did not recognize such a root in Ambly- 

 stoma, nor have I seen its peripheral course, though I have some 

 evidence of its presence (see p. 361). 



The motor roots of the V nerve emerge ventrally of the sensory- 

 roots; those of the VII nerve ventrally of all sensory VII and VIII 

 roots and partly ventrally of the spinal V root and partly through 

 it; the motor IX root fibers pierce the VIII roots; and the motor 

 X rootlets emerge at various levels. The more rostral of these 

 motor vagal roots emerge above the spinal V root, the middle ones 

 through this root and the more caudal ones below it. 



In the 38 mm. larva from which figures 1 to 18 are drawn there 

 are seven distinct roots of the vagus. The first includes the dorsal 

 and ventral lateral line roots, which enter the oblongata close 

 together at the same transverse plane as the IX roots (fig. 3). 

 The second and third roots contain visceral sfensory and visceral 

 motor components. These two roots constitute the second root of 

 Coghill ('02, p. 233). The fourth and fifth roots contain visceral 

 sensory fibers (entering the fasciculus solitarius) , somatic sensory 

 fibers (entering the spinal V root) and visceral motor fibers; these 

 are Coghill's third root. The sixth and seventh roots are exclu- 

 sively visceral motor and might be further subdivided. They 

 form Coghill's fourth root, which he says may arise by as many as 

 five distinct rootlets. 



Figure 3 shows diagrammatically the central courses of the 

 sensory components of the V to X cranial nerves as projected 

 upon the lateral surface of a model of the medulla oblongata. 

 This arrangement is preserved without essential modification in 

 the adult, though with some changes in the form relations and 

 much additional complexity of detail. 



Examination of the model from above (fig. 1) shows a configura- 

 tion not unlike that which I have described ('14) in adult Nec- 

 turus. The wide rhomboidal fossa is contracted in the upper 

 vagal region; passing from this region forward, it gradually 

 expands in the regions of the IX, VIII and VII nerves; and be- 

 tween the V roots and the cerebellum abruptly dilates to form the 



