740 THE NERVOUS SYSTEM. 



If the pia mater covering the hemisphere on each side of that covering the 

 inferior worm (the lateral extension of the same layer) be traced, it will be found 

 to follow a similar course, thus : After passing upward it reaches the under sur- 

 face, and then the free edge, of the lateral part of the inferior velum. It is now 

 sharply reflected on itself, and passes downward on the dorsal aspect of the resti- 

 form body. This is merely the lateral extension of the pia matral portion of the 

 tela choroidea inferior. As this layer passes from the edge of the inferior velum 

 (lateral part) to the restiform body it is in contact, just at its downward bend, 

 with the epithelium of the lateral recess just where it is being reflected on to the 

 extreme upper end of the restiform body. It is therefore evident that along the 

 free edge of the entire inferior medullary velum there is attached a margin of pia 

 mater consisting of two layers folded on themselves. For some distance down- 

 ward these two layers are somewhat adherent to each other. 



The Ligula. In the fourth ventricle, as well as in the other ventricles, the 

 lining epithelium has between it and the surrounding brain-tissue a delicate layer 

 of neuroglia known as the ependyma. The upper surface of the inferior velum 

 also has on it some of this ependyma. At the free edge of the velum, however, 

 this ependyma ceases abruptly, and the epithelium comes in direct contact with 

 the pia mater. But where the epithelium leaves the pia mater to be reflected 

 upward on the restiform body (lateral recess), or where it leaves the under sur- 

 face of the tela choroidea inferior to be reflected on the inner aspect of the resti- 

 form body and floor of the ventricle, the ependyma reappears. Hence this edge 

 of ependyma follows the line of reflection of epithelium all the way from the 

 calamus scriptorius obliquely upward and outward, and then outward around the 

 upper end of the restiform body. It is not visible to the naked eye until the pia 

 mater is pulled away. This tearing away of the pia mater necessarily brings 

 with it the epithelium lining it, and there is seen a very delicate, jagged line of 

 tissue following the course of the "line of reflection " just described. This line 

 of tissue is the edge of the ependyma plus the torn edge of epithelium, and is the 

 ligula, not to be confounded with the lingula. The union of the lower ends of 

 the two ligulse in the calamus scriptorius is known as the obex. 



This tearing away of the pia mater and epithelium also brings into view the 

 free edge of the inferior medullary velum, especially its lateral portions on each 

 side of the nodulus. 



Choroid Plexuses. The under surface of the pia-matral portion of the tela 

 choroidea inferior is not smooth. It has a linear series, on each side of the mid- 

 dle line, of minute vascular tufts of pia-mater tissue projecting ventrally. These 

 tufts, however, do not pierce the epithelial "roof," but are covered, ventrally, 

 everywhere by the epithelium. These lines of tufts with their epithelium are the 

 choroid plexuses of the fourth ventricle. 



Similar, but longer, tufts of pia-mater tissue are also prolonged from the pia 

 mater which lies beneath the epithelium, closing in the lateral recess. These last 

 are collected into quite a distinct bunch, resembling a group of small villi, which 

 is seen between the flocculus and outer aspect of the restiform body. In the 

 middle of these villi is the pointed cul-de-sac of the lateral recess, which by some 

 observers is said to be perforated by a minute foramen. 



A similar foramen is said to be present in the tela choroidea inferior just above 

 the calamus scriptorius. This is the so-called foramen of Majendie. 



THE MID-BRAIN (Figs. 415, 416, and 439). 



The mid-brain, or mesencephalon, is that portion of the brain which connects 

 the pons Varolii below with the cerebrum (inter-brain and hemispheres) above. On 

 this account it is sometimes called the isthmus or the cms cerebri. It has four 

 surfaces a superior, an inferior, a dorsal, and a ventral. The first two are flat- 

 tened and are attached, the superior to the cerebrum, the inferior to the pons. 

 They are also nearly parallel with each other. The two latter are somewhat curved 



