532 NERVOUS SYSTEM. 



Upon the upper surface of either hemisphere there are two lobes, separated from 

 each other by a fissure. These are the anterior or square lobe, which extends 

 as far back as the posterior edge of the vermiform process, and the posterior or 

 semilunar lobe, which passes from the termination of the preceding to the great 

 horizontal fissure. 



Upon the under surface of either hemisphere there are five lobes, separated by 

 sulci ; these may be described from before backwards. The flocculus or sub-pedun- 

 cular lobe is a prominent tuft, situated behind and below the middle peduncle of the 

 cerebellum ; its surface is comppsed of gray matter, subdivided into a few small 

 laminae ; it is sometimes called the pneumo gastric lobule, from being situated be- 

 hind the pneumogastric nerve. The amygdala or tonsil is situated on either side 

 of the great median fissure or valley, and projects into the fourth ventricle. The 

 digastric lobe is situated on the outside of the tonsil, being connected in part with 

 the pyramid. Behind the digastric is the slender lobe, which is connected with 

 the back part of the pyramid and the commissura brevis ; and more posteriorly 

 is the inferior posterior lobe, which also joins the commissura brevis in the valley. 



FOURTH VENTRICLE. 



The fourth ventricle or ventricle of the cerebellum is the space between the pos- 

 terior surface of the medulla oblongata and pons in front, and the cerebellum 

 behind. It is lozenge-shaped, being contracted above and below, and broadest 

 across its central part. It is bounded laterally by the processus e cerebello ad testes 

 above, and by the diverging posterior pyramids and restiform bodies below. 



The roof is arched; it is formed by the valve of Vieussens and the under surface 

 of the cerebellum, which presents in this situation four small eminences or lobules, 

 two occupying the median line, the nodulus and uvula, the remaining two, the 

 amygdalae, being placed on either side of the uvula 



The anterior boundary or floor is formed by the posterior surface of the medulla 

 oblongata and pons. In the median line is seen the posterior median fissure ; it 

 becomes gradually obliterated above, and terminates below in the point of the 

 calamus scriptorius, formed by the convergence of the posterior pyramids. At this 

 point is the orifice of a short canal terminating in a cul-de-sac, the remains of the 

 canal which extends in foetal life through the centre of the cord: On each side 

 of the median fissure are two slightly convex longitudinal eminences, the fasciculi 

 teretes ; they extend the entire length of the floor, being indistinct below, and of 

 a grayish color, but well marked and whitish above. Each eminence consists of 

 fibres derived from the lateral tract and restiform body, which ascend to the cere- 

 brum. Opposite the crus cerebelli, on the outer side of the fasciculi teretes, is a 

 small eminence of dark gray substance, which presents a bluish tint through the 

 thin stratum covering it ; this is called the locus coeruleus; and a thin streak of the 

 same color continued up from this on either side of the fasciculi teretes, as far as 

 the top of the ventricle, is called the tsenia violacea. The lower part of the floor 

 of the ventricle is crossed by several white transverse lines, linese transversee; they 

 emerge from the posterior median fissure ; some enter the crus cerebelli, others 

 enter the roots of origin of the auditory nerve, whilst some pass upwards and 

 outwards on the floor of the ventricle. 



The lining membrane of the fourth ventricle is continuous with that of the 

 third, through the aqueduct of Sylvius, and its cavity communicates below with 

 the sub-arachnoid space of the brain and cord through an aperture in the layer of 

 pia mater, extending between the cerebellum and medulla oblongata. Laterally, 

 this membrane is reflected outwards a short distance between the cerebellum and 

 medulla. 



The choroid plexuses of the fourth ventricle are two in number; they are 

 delicate vascular fringes, which project into the ventricle on each side, passing 

 from the point of the inferior vermiform process to the outer margin of the resti- 

 form bodies. 



