CENTERS OF CORRELATION. 253 



CHAPTER XVI. 



CENTERS OF CORRELATION (Continued). THE MESEN- 

 CEPHALON AND DIENCEPHALON. 



I. THE CUTANEOUS APPARATUS. 



In describing the centers and fiber tracts connected with the 

 general and special cutaneous nerves (Chapters VI and VII) it 

 was shown that the largest secondary tract from the primary 

 centers goes in the form of internal arcuate fibers to the opposite 

 side of the brain and passes forward to end in the roof of the 

 mesencephalon. This tract is called in lower vertebrates the 

 tractus bulbo-tectalis, in mammals the lemniscus. The tract 

 has a wider distribution in the mesencephalon and diencephalon 

 in mammals than in lower vertebrates. All the fibers which have 

 this general course may be known as the lemniscus system. The 

 evolution of this system and of its end nuclei will be considered 

 here. 



The first important point to notice is that the lemniscus system 

 in mammals is divided into two chief parts, the medial and the 

 lateral lemniscus. Each of these parts is believed to contain 

 descending fibers in addition to the ascending ones. It must be 

 emphasized that the ascending fibers alone are of interest in the 

 present connection. The medial lemniscus arises in small part 

 from the dorsal horns of the cord and chiefly from the nuclei of 

 the dorsal funiculi and from the nucleus of the spinal V tract. 

 It is therefore a general cutaneous conduction path. Some ascend- 

 ing fibers from the nuclei of the vestibular nerve probably join 

 the medial lemniscus and others probably run separately to 

 similar destinations in the diencephalon. The lateral lemniscus 

 arises from the various nuclei of the cochlear nerve and is therefore 

 an auditory conduction path. The chief place of ending of the 

 lateral lemniscus is the posterior corpora quadrigemina, that of 



