294 ANATOMY OF THE DOMESTIC FOWL 



light. The marked coloring is said to be most distinct in the 

 second layer. 



The anterior portion of the cerebellum consists of medullated 

 nerves whose fibers rim crosswise. These are the extensions of the 

 true cerebellar fibers, which become fewer and fewer as they proceed 

 anteriorly. Finally, a few remaining fibers running crosswise fuse 

 as the commissura Sylvii, and extend into the lobi optici. A layer 

 of nerve cells extends under these medullated nerve fibers. 



A bundle of fibers extending from the medulla oblongata. These 

 fibers extend into the optic lobes, form the criu^a cerebri, or 

 pedimcles of the cerebri, and also form the inferior waU, or floor, 

 of the aqueduct of Sylvius. The crura are sometimes spoken of as 

 the partes pedunculares. These contain gray matter. The follow- 

 ing division is made of the ganghon of this region. First, a group of 

 ceUs near the base of the peduncles, which are divided from the 

 ventral rim by the arciform fibers. This gangUonic formation may 

 be considered an extension from the medulla oblongata. Second, 

 a group of the ganglion cells of the lobus optici. Third, a group 

 of cells lying near the lobus opticus, which give rise to the ascending 

 roots of the trigeminus. Fourth, a group of gangUonic cells from 

 which spring the roots of the motores occuh, and which Hes near 

 the middle line and under the sulcus centralis. Many ganglion 

 cells are found to the right and to the left of the sulcus centralis, 

 which show different arrangement of the cells at different levels. 

 Fifth, a group of irregular cells which extends centrally on the 

 superior walls of the third ventricle and to the superior part of the 

 lobus opticus. 



The microscopic structure of the roof of the lobus opticus, or 

 corpus bigeminum, shows that the layers are arranged parallel to 

 the pia mater and are divided into the following parts: 



Externally, the pia mater. 



An outer layer of very fine nerve fibers, which hes just below the 

 pia mater. 



A layer of fine granular ground substance. 



A thin layer of small cells the diameter of each of which is 0.038 

 mm. 



A layer of fine granular ground substance. 



A second thin cell layer. 



A zone of fine granular ground substance. 



A third thin cell layer. 



