REGIONAL ANALYSIS 49 



hemisphere. Its efferent fibers go to the tectum, thalamus, hy- 

 pothalamus, and cerebral peduncle (figs. 11, 12, 14, 15, 16, 22, 23). 

 Its functions are unknown, but, by analogy with mammals, this may 

 be part of the apparatus for regulation of the intrinsic musculature of 

 the eyeball. Doubtless other functions are represented also. This area 

 is the probable precursor of the mammalian pulvinar and neighboring 

 structures. 



The thalamus receives many fibers from the retina, and it is broad- 

 ly connected with the tectum by uncrossed fibers passing in both 

 directions in the brachia of the superior and inferior colliculi (figs. 11, 

 12). There are also systems of tecto-thalamic and hypothalamic and 

 thalamo-hypothalamic tracts which decussate in the postoptic com- 

 missure ; some of these crossed fibers take longer courses to reach the 

 peduncle and isthmic tegmentum (figs. 12, 15). This intimate 

 thalamo-tectal relationship is radically changed in higher animals, 

 where the thalamo-cortical connections are highly elaborated. 



8. DORSAL THAL.\MUS 



I have separated the dorsal thalamus into three sectors: (1) an- 

 teriorly, the small nucleus of Bellonci of uncertain relationships ; (2) a 

 well-defined middle part, an undifferentiated nucleus sensitivus, 

 which is the primordium of most of the sensory nuclei of the mam- 

 malian thalamus; and (3) a vaguely delimited posterior sector, which 

 apparently contains the undifferentiated primordium of both lateral 

 and medial geniculate bodies (chap. xvii). 



The middle and posterior sectors receive numerous terminals and 

 collaterals of the optic tracts, terminals of the general bulbar and 

 spinal lemnisci, and, through the brachia of the superior and inferior 

 colliculi, these sectors are broadly connected with the tectum by 

 fibers running in both directions. There is a similar, but much 

 smaller, connection with the habenula. 



From the middle sector a small, well-defined tr. thalamo-frontalis 

 goes forward to the hemisphere (figs. 15, 71, 72, 95, 101, tr.th.f.); 

 this is the common primordium of all the thalamo-cortical projection 

 systems of mammals, though here few, if any, of its fibers reach the 

 pallial area. Other efferent fibers go to the ventral thalamus, hy- 

 pothalamus, peduncle, and tegmentum. These thalamic reflex con- 

 nections antedate in phylogeny the thalamo-cortical connections, 

 and they persist in mammals as an intrinsic paleothalamic apparatus, 

 an important part of which is the periventricular thalamic contribu- 

 tion to the f. longitudinalis dorsalis of Schiitz. The largest pathwaj^s 



