258 NERVOUS SYSTEM OF VERTEBRATES. 



essentials similar to that in amphibia. The wall has thickened 

 and the anterior and posterior corpora quadrigemina have differ- 

 entiated. The anterior corpus quadrigeminum serves as the 

 place of ending of optic and secondary cutaneous tracts and is 

 comparable to the cephalic part of both tectum opticum and 

 colliculus in fishes. The posterior corpus quadrigeminum serves 

 as the chief place of ending of the secondary auditory paths and is 

 roughly comparable to the caudal part of the colliculus in fishes. 

 In birds a large tectum opticum has been developed on account 

 of the large size of the eyes and the mesencephalon of birds is 

 more like that of bony fishes than that of other lower vertebrates. 



The lateral lemniscus arises, as has been said, from the nuclei 

 of the cochlear nerve. A large part of the fibers come as internal 

 arcuate fibers from the nuclei of the opposite side, the remainder 

 as direct fibers from the nuclei of the same side. The lateral 

 lemniscus passes forward into the mesencephalon, bends upward 

 and separates from the medial lemniscus, and the greater part 

 of it enters the posterior corpus quadrigeminum. A part of the 

 fibers pass forward to end in the anterior quadrigeminum and a 

 part go on to end in the medial corpus geniculatum or adjacent 

 nuclei in the thalamus. From the posterior corpus quadrigeminum 

 fibers go to the medial corpus geniculatum and from this center 

 auditory impulses are forwarded to the cerebral hemispheres. 

 From the posterior quadrigeminum other fibers go down through 

 the lateral lemniscus to end in the various cochlear nuclei (recur- 

 rent fibers). 



The medial lemniscus consists of both crossed and uncrossed 

 fibers from the centers for cutaneous nerves. The tract receives 

 fibers also from many other sources not related to the cutaneous 

 nerves. As the tract passes forward fibers go from it to end in 

 the gray matter of the medulla oblongata, pons, isthmus, midbrain 

 and hypothalamus. It is shown, however, that the great majority 

 of the fibers which arise in the cutaneous nuclei pass forward to 

 the thalamus and it is believed that the other fibers which enter 

 and leave the tract do not belong to the lemniscus system proper. 

 The secondary cutaneous fibers end in the so-called nucleus 

 ventralis of the thalamus. It must be noticed that this nucleus 



