140 THE BRAIN OF THE TIGER SALAMANDER 



cerned chiefly with proprioceptive functions of the skin and deep 

 tissues of the head, as indicated by its strong cerebellar connection. 

 This connection persists in mammals but is relatively insignificant 

 here because, as mentioned above, the enlarged mammalian superior 

 V nucleus is concerned chiefly with refined functions of the skin that 

 Amblystoma does not possess. 



MESENCEPHALIC NUCLEUS AND ROOT OF THE TRIGEMINUS 



The system of the mesencephalic nucleus and root of the trigemi- 

 nus is here well developed in typical relations, with some instructive 

 special features. Its thick, well-myelinated fibers go out with 

 branches of the V nerve. The details of their peripheral courses in 

 Amblystoma have not been described. Experiments by Piatt ('46) 

 indicate that the majority of the fibers of this system, which go out 

 from the tectum opticum, are distributed to the jaw muscles. The 

 more caudal cells of the mesencephalic nucleus probably have other 

 connections. No evidence has been found for supply of any eye 

 muscles from this nucleus. 



Unlike other sensory systems, the cell bodies of these neurons lie 

 within the brain. Their arrangement and the courses of the fibers 

 arising from them have been described in the larva ('14a, p. 361) and 

 in the adult ('36, p. 345) and are shown diagrammatically in figure 

 13. These cells vary in size, cytological structure, and number. Most 

 of them are very large and of so characteristic appearance that they 

 are easily recognized. They are sparsely distributed throughout the 

 tectum in all layers of the gray substance, somewhat less numerous 

 anteriorly near the posterior commissure, and densely crowded with- 

 in and adjoining the anterior medullary velum. Occasionally, they 

 are seen in the body of the cerebellum and in the nucleus cerebelli. 

 Ten large larvae of A. punctatum had an average of 159 of these 

 cells, the extremes being 76 and 208 (Piatt, '45). A subsequent count 

 by Piatt ('46) of the total number of these cells in ten larvae of 45 

 mm. gave an average of 261 cells, equally divided on right and left 

 sides. Of these cells, 86 on each side are in the tectum opticum and 45 

 in the nucleus posterior tecti and velum medullare anterius. Individ- 

 ual variations in numbers of cells are large, but approximate bilateral 

 symmetry is quite consistently present. These cells are unipolar, the 

 single thick processes accumulating near the outer border of the 

 tectal gray and here acquiring myelin sheaths. These fibers are di- 



