NEUROLOGY. 117 



extremity, from whicli it is distinguished by a deep groove. Its surface is 

 white and fibrous : the superficial layers of fibres on its inferior surface 

 run transversely from the inferior surface of one crus cerebelli to the 

 other. 



4. The Cerebellum. The cerebellum is seen on removing the posterior 

 lobes of the cerebrum and dividing the tentorium. In size and weight it 

 bears to the cerebrum a ratio of about as one to eight ; the average weight 

 of the cerebrum being two pounds and a half, that of the cerebellum amount- 

 ing to about four ounces and a half. It is somewhat different in form from 

 the cerebrum, being oval trans versel}^, and raised in the centre, where its 

 right and left hemispheres are united. Like the cerebrum it is composed 

 of white substance internally and of grey upon its surface, upon which it is 

 marked by a great number of parallel narrow lines, running semicircularly 

 convex posteriorly ; these are fissures to the bottom of which the pia mater 

 descends, the arachnoid membrane passing over them. Some lines called 

 primary pass very deep into the cerebellum, and divide it into lobes ; 

 secondary lines, more superficial, divide these into lobules. 



The cerebellum presents for examination a superior and inferior surface, 

 a convex border or circumference, and a median notch behind and before. 

 The posterior notch is very deep. It receives the falx cerebelli and the 

 inferior occipital crest, and extends close along the under surface as far as 

 the back of the medulla oblongata. This extension is called the valley ov 

 the purse-like fissure. The anterior notch is broad, overlaps the fourth 

 ventricle, and embraces the central protuberance and tubercula quadrige- 

 mina. These two notches mark a division of the cerebellum into right and 

 left hemispheres, the circumference of each of which is deeply indented by 

 the hor izon ta I fissure. 



The sujierior surface of the median portion of the cerebellum is known 

 as the sujyerior vermiform pt^'ocess. Its anterior terminal laminas form the 

 valve of Yieussens. On the inferior surface the hemispheres are much 

 more convex than on the superior. The median portion consists of a series 

 of lamiuse following a transverse direction, those in the centre being of 

 greater transverse extent than those at either extremity, and constituting 

 the inferior veiiniform process. These two processes, which have thus 

 received distinct names, are really but one, and might be properly termed 

 the median or primitive lobe of the cerebellum. In the lower orders of 

 vertebrata this median lobe alone exists, the lateral portions increasing with 

 the ascent in the organic scale. 



The cerebellum may tlierefore be considered as divided into the median 

 lobe and two lateral hemispheres. The hemispheres present the lines or 

 sulci already referred to, on their superior surface, dividing them into lobes 

 and lobules. We can only briefly indicate these divisions in a general 

 survey of the entire cerebellum as follows : 



a. The cerebellum is divided into three parts, viz. riglit and left hemi- 

 spheres, and middle or primary portion, o. Each of these is subdivided by 

 the horizontal groove into superior and inferior, c. The upper surface of 

 each hemisphere presents two lobes, the anterior or square, and the posterior 



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