2OO 



THE NERVOUS SYSTEM 



important and constant fissure. It extends in a continuous curved line across the rostral 

 aspect. of the vermis and both hemispheres. It has been found by Ingvar (1918) in reptiles 

 and birds. All investigators who have given attention to this subject in recent years agree 

 in designating the portion of the cerebellum which lies rostral to the fissura prima as the 

 anterior lobe. The portion behind this fissure is composed of several individual lobules, each 

 of which, though subject to considerable variation in form in the different genera, can be 

 identified in every mammalian cerebellum. These lobules have been variously grouped into 

 lobes by different investigators. Here we will follow the grouping employed by Ingvar, which 

 is based on a comparison of the mammalian cerebellum with that of birds and reptiles (Fig. 

 142). He recognizes three major divisions of the cerebellum, which he designates as the 

 anterior, middle, and posterior lobes. The middle lobe contains those parts of the cerebellum 

 which have been the last to appear during phyletic development, and it is here that the 

 greatest variations are found in the different orders of mammals. 



1. 



Fig. 142. Schematic drawing of the cerebellum of 1, lizard; 2, crocodile; 3, bird, and 4, 

 mammal. Vertical lines, anterior lobe; stipple, middle lobe; horizontal lines, posterior lobe; white, 

 lobus ansoparamedianus. (Ingvar.) 



The anterior lobe includes all that part of the cerebellum that lies on the rostral side of 

 the fissura prima (Figs. 143, 144, 146). In this lobe the folia have a transverse direction and 

 extend without interruption across the vermis into both hemispheres. In the sheep the an- 

 terior lobe is bounded laterally by the parafloccular fissure. It includes the three most 

 rostral lobules of the superior vermis, which are designated in order from before backward, the 

 lingula, lobulus centralis, and culmen monticuli. In man it also includes a large wing-shaped 

 portion of each hemisphere (the pars anterior lobuli quadrangularis) ; and the entire lobe has 

 the shape of a butterfly (Fig. 146). Morphologically, it is a median unpaired structure. 



The middle lobe is subdivided into four parts (Fig. 142). The most rostral of these 

 is the lobulus simplex. It is separated from the anterior lobe by the fissura prima, and like 

 that lobe it consists of transverse folia which extend across the superior vermis into both 



