SPINAL NERVES IN VERTEBRATES 147 



of the spinal ganglion of this nerve. The cephalic ventral root 

 (V.R (1) ) takes origin from two rootlets, the first, which is not 

 figured, leaves the latero-ventral surface of the spinal cord a 

 little behind the level of the preceding spinal ganglion. The 

 course of the cephalic root is at first caudad within the neural 

 arch. Its exit through the membranous neural ' arch lies be- 

 tween the two dotted lines in figure 5. At least five rootlets 

 take part in the formation of the caudal ventral root (V.R. {2) ). 

 Two of the more cephalic rootlets pursue a caudo-lateral course 

 and unite in a common stem which passes through a foramen in 

 the neural arch, situated on a level with the dorsal surface of 

 the notochord and about equidistant between the foramen for 

 the cephalic ventral root and the spinal ganglion. Three of the 

 most caudal rootlets travel cephalad and laterad to join in a 

 common stem, which leaves the neural arch through a foramen 

 immediately behind the common stem formed from the union 

 of the two preceding rootlets. Directly outside their foramina 

 these two motor stems unite, forming a caudal ventral root, 

 which assumes a general ventral course to unite with the cephalic 

 ventral root a short distance cephalad of the lower corner of 

 the spinal ganglion. The short common trunk thus formed, 

 is the ventral motor ramus {E.V.M.). Its fibers soon inter- 

 mingle with those of the ventral sensory ramus, lateral and a 

 little below the level of the central axis of the notochord. From 

 the above description it will be seen that the motor components 

 of this nerve pierce the membranous neural arch through there 

 separate foramina. Through the first, the cephalic ventral 

 root fibers emerge, and through the second and third, motor 

 bundles leave to unite outside the neural arch in forming the 

 caudal ventral root. It is also clear that all of the motor com- 

 ponents are situated in front of the sensory. The mixed ramus 

 ventralis {R.V.) presents no special peculiarities except for the 

 presence of a distal ganglion cell noted in the pre\'ious para- 

 graph. In this nerve the dorsal motor ramus (R.D.M.) has a 

 double origin, differing in this respect from the more cephalic 

 nerves in figure 6. A bundle of fibers branches off from the 

 upper surface of the cephalic ventral root close to its exit from 



