Johnston, The Radix Mesencephahca Trigemini. 599 



The third of these evidences will be dealt with in the present paper. 

 The first, second and fourth, although fully accepted, are not in them- 

 selves conclusive evidence that the root is motor. The fact that fibers 

 of a peripheral nerve arise from cells situated within the central ner- 

 vous system is not evidence at all that the fibers are motor fibers. The 

 only conclusive evidence that the fibers of a bundle are motor is to 

 trace them continuously to motor end plates within a muscle or to 

 demonstrate their action physiologically. The origin of fibers from 

 central cells is not even presumptive evidence that the fibers are motor 

 unless the cells lie in the ventral (motor) zone of the cord or brain. 

 The cells in question lie in the roof. A large part of the ganglion 

 cells of the dorsal sensory nerves in Amphioxus lie within the central 

 nerve cord (Johnston, 1905) and in fishes and amphibians many cells 

 are found in the dorsal zone of the spinal cord and in some cases in 

 the medulla oblongata which give rise to sensory fibers of the dorsal 

 nerve roots. Among the many studies of these so-called giant cells, 

 cells of Rohon, etc., I need only refer to the papers by Van Gehuchten 

 (1897) and the writer (1900). In view of these well known facts, 

 the most natural assumption is that peripheral fibers arising from cells 

 in the brain roof would be sensory fibers as in all lower vertebrates. 



The fact that the fibers of this root degenerate downward after 

 lesion of the tectum is evidence only that they arise from cells in the 

 tectum and not that they are motor in function. 



The degeneration of this root after cutting the motor rami of the 

 trigeminus is much the strongest evidence thus far brought forward. 

 The weak point in the argument is that the motor rami contain sen- 

 sory fibers also and hence the peripheral operation is inconclusive. 

 The mesencephalic root may supply sensory fibers to the mixed rami. 



Obseevatiot^s, 

 In 1905 the writer described the radix mesencephalica in Scyllium 

 canicula, Acipenser and ISTecturus, showing that in each of these the 

 bundle joined the sensory root of the trigeminus. Since then the 

 bundle has been studied in Scyllium stellare, Squalus acanthias, 

 Cryptobranchus, the common toad, a turtle, the mole, rabbit, rat, 

 mouse, cat, pig embryos, one human embryo of 15.5 mm. and in 



