NEUROLOGY. 117 
extremity, from which 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 transversely, 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 semicireularly 
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 or 
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 horizontal fissure. 
The superior surface of the median portion of the cerebellum is known 
as the superior vernuform process. Its anterior terminal laminz form the 
valve of Vieussens. On the inferior surface the hemispheres are much 
more convex than on the superior. The median portion consists of a series 
-of laminz following a transverse direction, those in the centre being of 
greater transverse extent than those at either extremity, and constituting 
the inferior vernuform 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 therefore 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. right and left hemi- 
spheres, and middle or primary portion. 6. 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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