133 ANATOMY OF THE CENTBAL NEETOUS SYSTEM. 



other side. The fasciculus and the corpus interpedunculare are absolutely 

 constant throughout the whole vertebrate series. 



The ganglia peculiar to the thalamus may only be sharply difEerentiated 

 from those of the hypothalamus in the mammals and birds; in lower verte- 

 brates the two regions merge into one another. 



To the author the Thalamus of the reptile is best known, and, since 

 the transition from this to the same organ of birds and mammals may be 

 readily made, it will be advisable to begin with a close study of the former. 



Anteriorly there enters into the thalamus from the corpus striatum of 

 the cerebrum a large bundle: Tr. sirio-thalamicus. The bundle splits up in 

 such a manner that each one of the ganglia to be named immediately re- 

 ceives fibers which, separated from the others, pass to it direct. This char- 

 acterizes all the thalamic ganglia. 



The nucleus which strikes one first is a large, round one, composed of 

 immense multipolar cells, — the Nucleus rotundus thalami. It is foimd like- 

 wise in birds, here with a retort-shaped, posteriorly directed projection. In 

 fishes, also, it is a very large ganglion. In the nucleus rotundus the bundles 

 of the Tr. strio-thalamiciis split up into fine rays (Fig. 81), and from it 

 arises a fasciculus which, passing backward, ends in the roof of the mid- 

 brain: Tr. thalamo-tectalis (Fig. 83). The appearance of this nucleus and 

 its connections are especially characteristic and very constant. 



Somewhat anterior and dorsal to the Nuc. rotundus in the neighbor- 

 hood of the Ggl. habenulse lies the Nuc. anterior (Figs. 70 and 83). From 

 it arises a characteristic bundle, — characteristic of reptiles, birds, and mam- 

 mals, — which, traversing the whole thalamus quite parallel to the Tr. 

 habenulo-peduncularis, passes to the base and is lost in the Corpus mam- 

 millare located there: Tr. thalamo-mammillaris (Fig. 83), — in mammals 

 called the fasciculus of Vicq d'Azyr. 



Ventral to the two described nuclei one finds in reptiles and birds, 

 probably also in fishes, in the midst of the gray matter that surrounds the 

 median ventricle, an elongated nucleus of large cells: Nuc. magno-cellularis 

 strati grisei. It is probably fibers from it that pass ventrally to cross just 

 over the infundibulum as the Decussatio supra-infundibularis. Besides this, 

 it probably sends bundles off posteriorly. 



In many reptiles, especially turtles, in mammals, and possibly in birds, 

 the two sides of the ventricular wall fuse together for a little distance in 

 the midst of the thalamus. Thus arises the Gommissura grisea thalami: the 

 Commissura mollis in mammals. In crocodiles, turtles, and snakes there lies 

 here a large nucleus, — median nucleus (Fig. 82), — the connections of whose 

 fibers arc not yet clear. At any rate, it receives fibers from the striatum and 

 sends fibers out laterally. The median nucleus is not sharply defined, but 

 there lie in the gray matter of the thalamus, some near to the median 



