282 C. JUDSON HERKICK 



surface of the stratum griseum is a similar layer of tangential 

 fibers, chiefly unmyelinated, which carry nervous impulses from 

 the tectum to the underlying cerebral peduncle and motor 

 tegmentum (tractus tecto-peduncularis profundus, see p. 254). 



The histological organization of the tectum exhibits a very 

 primitive pattern with very imperfect functional localization. 

 My preparations reveal no important structural differences 

 between the optic and the somesthetic parts of the tectum, 

 except that the walls of the latter are somewhat thicker and the 

 neuropil of the stratum album is more dense (figs. 9 to 14). 

 The individual neurons of the tectum, so far as shown by our 

 preparations, are of several types, of which the one most com- 

 monly impregnated in our Golgi sections has a single thick 

 dendrite which passes through the stratum griseum and branches 

 as soon as the stratum album is reached (figs. 21, 22, 23, 31, 32, 

 33, 35). These branches are thick, contorted and very widely 

 spread throughout the stratum album, frequently ending in 

 dense irregular tufts approaching the form of glomeruli (figs. 

 22, 23). They differ in this respect from the large and equally 

 widely spread dendrites of the neurons of the pedunculus cere- 

 bri, which are smoother and more diffusely branched at their 

 termini (fig. 23). The axon usually springs from one of the 

 larger dendrites near its base in the deeper levels of the stratum 

 album (fig. 31, ax.) These axons are impregnated for a very 

 short distance and probably acquire myelin sheaths at once. 

 The dendrites of a single neuron of this type may spread 

 throughout the entire dorso-ventral extent of the tectum and 

 thus effect functional connections with diverse afferent systems. 

 Figures 21, 22, 23, 32 illustrate neurons, some of whose den- 

 drites arborize in the terminal area of the optic tract, while others 

 reach only the terminals of the lemniscus systems. An element 

 of this type may, therefore, receive either optic or tactile stimuli 

 or both of these simultaneously, this being the simplest possible 

 type of a correlation center. Other neurons l>dng farther ven- 

 trally in the tectum may send one large dendrite into the terminal 

 area of the lemniscus (colliculus inferior) and another into the 

 tegmentum (figs. 31, 32), thus being able to respond to excita- 



