616 PROCEEDINGS OF THE AMERICAN ACADEMY. 



(each leaving the nerve cord by a separate orifice), which unite into 

 a flat bundle; this soon spreads out, a small portion going dorsad, 

 and a larger part ventrad. When the fibers reach the muscle layer 

 they bend posteriorly and coalesce with the free margin of the muscle- 

 plate. As the fibers approach the border of the muscle-plate, they 

 become cross-striated. Schneider proposed the theory that the real 

 motor nerves extend outward only as far as the sheath of the spinal 

 cord, and that each muscle-plate sends a process to the spinal cord 

 and receives its innervation there. Balfour ('80) denies the presence 

 of motor roots. Rohon ('82, p. 14, 54) states that the ventral roots 

 arise from multipolar ganglion cells, in part pigmented, which lie on 

 both sides of, and in part ventral to, the central canal of the cord. 

 He finds no morphological relation between dorsal and ventral roots 

 outside the spinal cord and is entirely in the dark as to how the motor 

 nerves end. Roh.de ('88) thinks, with Schneider, that the motor 

 fibers are probably in part direct processes of the muscle fibrillae ; but 

 he is uncertain whether there is a direct connection between the motor 

 fibers and the nerve elements of the neural tube. Fibers in the ventral 

 roots were followed centripetally to a delicate membrane on the inner 

 side of the corda sheath, where they were believed to divide into 

 very fine fibers, which, however, could not be traced in the spinal cord. 

 Fusari ('89) finds, as did Schneider and Rohde, most of the ventral- 

 root fibers cross-striated; but some of them are not striated, exhibit 

 enlargements, and stain black by the Golgi method. Retzius ('91) 

 notes in each ventral root an anterior division composed only of 

 varicose fibers, and a posterior, somewhat smxaller di\'ision carrying 

 fibers mostly without varicosities. Each fiber of the posterior por- 

 tion bears an oval body a short distance after its exit from the spinal 

 cord. Retzius is unable to decide whether these are sheath nuclei, 

 or of some other nature. Toward the spinal cord motor nerve fibers 

 were followed singly into the outer layer of the cord, where they enter 

 a low, granular mound; but beyond this they cannot be followed. 

 In these mounds the fibers often bend in hook-fashion. Retzius thinks 

 that the connection of motor fibers with central ganglion cells, which 

 he does not doubt, cannot be very direct. Fibers occasionally divide 

 dichotomously, but there are no special nerve endings. Especially 

 characteristic of these fibers is their varicose-granular condition. 

 Owing to the abundance and transverse elongation of the granules, 

 they give a cross-striated appearance to the fibers, probably the cause 

 of Schneider's erroneous views, for these are true nerve fibers, and not 

 of muscular nature. He is inclined to accept Rohde's suggestion that 



