148 NERVOUS SYSTEM OF VERTEBRATES. 



and branches very richly in the middle layers of the tectum in the 

 immediate vicinity of the cell. All the other cells of the tectum 

 may be described as cells of type I whose dendrites spread more 

 widely and do not reach the outer surface of the tectum. Some 

 of these cells have vertical spindle-shaped bodies and one or more 

 branching dendrites, others have stellate bodies with several den- 

 drites diverging widely, and still others are spindle-shaped but 

 are placed tangentially and have two dendrites running parallel 

 with the surface of the tectum. The neurites may arise from 

 the cell-bodies or from any part of the dendrites, even from 

 the tip of one of the tangential dendrites. The neurites arising 

 from the cells of type I have various courses and destinations, 

 (i) They cross to the opposite side, forming the dorsal decussation 

 of the tectum. Whether any or all of these fibers leave the tectum 

 after crossing is not known. (2) They enter the tractus tecto- 

 bulbaris et spinalis and go either to the same or opposite side of 

 the medulla oblongata and spinal cord. (3) They go down 

 through the lateral wall of the mesencephalon or diencephalon 

 to end in the inferior lobes, the tractus tecto-lobaris. Part of 

 these fibers cross to the opposite side in the ansulate commissure in 

 the ventral wall of the mesencephalon or in the postoptic decussa- 

 tion, or in both. (4) They go along the lateral border of the tec- 

 tum to enter the cerebellum, the tractus tecto-cerebellaris. (5) They 

 go out in the optic tract to end in the retina (Fig. 72). The last 

 are called centrifugal fibers. The elements in the optic lobe of 

 birds are shown in Fig. 74. 



A special apparatus is described in connection with the tectum 

 opticum of all classes of vertebrates which is supposed to serve 

 at least in early life for direct and prompt reflexes in response 

 to optic impulses. In the mesial part of the tectum is found a 

 nucleus consisting of a variable number of cells which are usually of 

 extraordinary size. This is known as the roof nucleus or nucleus 

 magnocellularis tecti. The neurites of these cells are said to enter 

 the ventricle and form the structure known as Reissner's fiber, 

 which makes connections with cells in the gray matter of the 

 spinal cord. By means of this apparatus it is supposed that 

 aquatic animals are able to avoid obstacles and danger by move- 



