202 



THE NERVOUS SYSTEM 



The posterior lobe, as outlined by Ingvar, is composed of median and lateral portions. 

 The median part, known as the posterior median lobule, comprises all of the inferior vermis 

 except the tuber, from which it is separated by the prepyramidal sulcus. It is subdivided 

 into three sublobules, known as the nodule, uvula, and pyramid (Figs. 139, 141, 145). The 

 lateral part of the posterior lobe is formed on either side by two lobules, known as the flocculus 

 and paraflocculus. These form the most lateral portion of the hemisphere in most mammals 

 (Figs. 142, 144). In man the paraflocculus is rudimentary and the flocculus lies upon the 

 caudal surface of the hemispheres (Fig. 147). It is connected with the nodule by a thin 

 sheet of white matter, the posterior medullary velum. 



Functional Localization in the Cerebellum. We have described the cerebellum in 

 terms of the subdivisions of Bolk and Ingvar, because these have morphologic and physio- 

 logic significance, which is not true of the parts into which the cerebellum had previously 

 been divided. By comparison of the size of these subdivisions with the degree of develop- 

 ment and functional importance of the various groups of muscles in different animals Bolk 

 endeavored to show that each of these parts was related to a particular group of muscles. 

 On the basis of these comparative studies he concluded that the median unpaired portions 

 of the cerebellum serve as coordination centers for the muscles which function in bilateral 



Tuber vermis 



Prepyramidal snlcus^ 



Paraflocculus -- 



-' Lobulus ansiformis 



Lobulus paramedianus 



~ Lobulus mcdianus posterior 



Fig. 145. Cerebellum of the sheep, caudal view. 



synergy. The muscles of expression and mastication, those of the eyes, pharynx, larynx 

 and neck, and many of the trunk muscles are called into action simultaneously on both sides 

 of the body, and should, according to this theory, have a median unpaired representation 

 in the cerebellum. Bolk located the coordination center for the musculature of the head 

 in the anterior lobe, that for the muscles of the neck in the lobulus simplex (Figs. 146, 147). 

 A median center for those movements of the extremities which are strictly bilateral is found 

 in the most dorsal sublobule of the vermis inferior, known as lobulus C 2 or tuber vermis. 

 The remainder of the inferior vermis forms, according to this theory, a center for the bilateral 

 movements of the trunk. In addition to a median center in the tuber vermis, the limbs are 

 represented in the cerebellum by lateral centers for the coordination of unilateral move- 

 ments. The lateral center for the arm is located in the rostral part or crus primum of the 

 lobulus ansiformis (superior and inferior semilunar lobules) and that for the legs in the caudal 

 part or crus secundum (biventral lobule), and perhaps also in the lobulus paramedianus 

 (tonsil) . 



The conclusions concerning the localization of function in the cerebellum, reached by 

 Bolk on the basis of morphologic studies, have been confirmed in so far as the centers for the 

 neck and extremities are concerned by animal experimentation (Van Rynberk, 1908, 1912; 



