136 ANATOMY OF THE CENTRAL NERVOUS SYSTEM. 



mammals with the previously described imclei of reptilia. It will require 

 much work yet before it is known what new structures have been introduced 

 and what is to be attributed only to the increase of structures already pres- 

 ent. As yet, it is not jDossible to determine more than that, in mammals, fibers 

 from the Tr. strio-tlialamici end in all or nearly all of the thalamic ganglia, 

 and that the Tr. thalamo-hiilbares et spiiudes are developed from one of the 

 ventral nuclei. 



Only a few thalamic nuclei of mammals may be homologized with those 

 of lower vertebrates; and these will be more minutely described, because, 

 in them, we have learned to recognize the whole process of the addition to 

 already existing systems of cerebral tracts which are not necessary to the 

 existence of lower vertebrates. First, there is the Corp. geniculatum. In 

 all animals fibers from the the optic nerve pass into it. Prom birds upward 

 in the vertebrate series there is formed a tract from the cerebral cortex to 

 the optic center. Whether in birds it reaches the geniculatum is not 

 known, but that it does so in mammals has been conclusively demonstrated. 

 So you see, anatomically recognizable, how the primitive terminal center 

 of the optic nerve passes, in higher animals, into relations with tracts which 

 arise in the organ of thoiight, of memory, of association, etc. 



For the ventral nucleus in which the tracts to the medulla and spinal 

 cord end, a similar relation has also been recognized in the mammals. Here 

 it receives afferent bundles from the cerebral cortex, indeed, from psycho- 

 motor areas: from cortical areas whose loss diminishes the ability to carry 

 out acquired movements or movements which are the result of associative 

 reflexes. 



Though these nuclei exist in lower vertebrates, it is only in the highest 

 of the series that the cerebral tracts are added. 



So much for the ganglia peculiar to the thalamus. There still remains 

 for consideration a narrow region that is usually reckoned in with it: the 

 boundary region between the interbrain and the midbrain,the metathala- 

 mus. 



Just anterior to the roof of the midbrain and continuous with it, simply 

 extending out into the thalamus anteriorly, we find the Nuc. proetectalis. 



It has not been demonstrated yet in mammals, though I believe that 

 it is to be recognized in the most anterior portion of the ganglion reckoned 

 until now as belonging to the gray matter of the anterior quadrigeminal 

 body. Of this nucleus something, has been said before (Figs. 71 and 72), 

 and also of the fact that in it probably bundles from the stilus of the gen- 

 iculatum end (Fig. 83). Median from it lies a not very sharply defined 

 nucleus, from which the bundles of the commissura posterior — Nuc. com- 

 missura posterioris — appear to develop, and ventral to it one finds, through- 

 out the whole vertebrate series, the nucleus of origin of the posterior 



