OPTIC LOBES OF AMBLYOPSIS. 



107 



(3) An optic cell layer. 



(4) A deep cell layer. According to Krause this layer contains in its outer part the cells 

 which serve as terminal stations for the optic nerve, and in its inner sublayer the end 

 stations for the fifth layer (Marklager). 



(5) A deep fiber layer. 



(6) A granular layer. 



(7) The ependyma and its epithelium, which lies next to the ventricle of the lobes. 



The optic lobes of Amblyopsis show a marked degeneration. The dorsal walls 

 are not more than half or two-thirds as thick as those in the normal brain. Its 

 contour is so flattened that the ventricle is almost obliterated (16 in fig. 34 b). The 

 torus longitudinalis, which in the normal brain is suspended in the ventricle in the 

 median line entirely below the layers of the lobes, is between the lobes and on nearly 

 the same level with them. The torus thus forms a commissure connecting the 

 lobes. The band of fibers connecting them dips downward in the normal brain 

 and crosses to the opposite side through the torus; in the degenerate lobe they 

 cross from one side to the other in almost a straight line (15 in fig. 34 b). The 

 shrinkage in length is shown in the fact that the hypophysis is crowded forward to 

 the anterior level of the lobes- 



^14 



Fig. 34. (a) Cross-section of Brain. Amblyopsis spelceus near Anterior part of Optic Lobes. Specimen 77 mm. long. 



(b) Cross-section through Middle of Optic Lobes of Amblyopsis spelceus. Specimen 77 mm. long. i, first 



layer of optic lobe; 2, degenerate optic fiber layer; 3, optic cell layer; 4, deep cell layer; 5, deep 



fiber layer; 5 a, diagonal fibers of deep fiber layer; 6, granulated layer; 7 a, optic tract region; 



13, ependyma; 15, torus longitudinalis; 16, ventricle. 



The optic nerve of the normal brain is derived from the second and fourth layers 

 of the lobes. The fibers of the second layer pass downward on both sides of the 

 lobes, and the inner ones cross over at the ventral surface, where they join the fibers 

 of the same layer from the other side. They then continue forward and downward 

 to the optic chiasma as the optic tracts. The fifth layer is composed of diagonal 

 fibers and descending fibers. These latter nerves pass downward and become a 

 part of the optic tract. 



As has been said, the wall of the optic lobes of Amblyopsis has undergone con- 

 siderable shrinkage in thickness. The outer layer is not changed. The second 

 layer, which is derived from the optic nerve, is entirely wanting. The optic nerve 

 is represented by a small bundle of tissue, which is probably the remnant of the 

 neurilemma. In the brain where the second layer should be, there is a narrow 

 space containing practically no tissue. The third layer is unchanged. The fourth 



