ANATOMY OF THE MESENCEPHALON I2Q 



This is supported on its outer surface by a layer of pia mater, the tela chorioidea, 

 rich in blood-vessels. From this layer vascular tufts, covered by epithelium, 

 are invaginated into the cavity and form the chorioid plexus of the fourth ven- 

 tricle (Fig. 90). The plexus is invaginated along two vertical lines close to the 

 median plane and along two horizontal lines, which diverge at right angles from 

 the vertical ones and run toward the lateral recesses. These right and left 

 halves are joined together at the angles so that the entire plexus has the shape 

 of the letter T, the vertical limb of which, however, is double. 



After the tela chorioidea with its epithelial lining has been torn away to 

 expose the floor of the ventricle, there remains attached to the lateral bound- 

 aries of the caudal part of the cavity the torn edges of this portion of the roof. 

 These appear as lines, the teenies of the fourth ventricle, which meet over the 

 caudal angle of the cavity in a thin triangular lamina, the obex (Fig. 89). Ros- 

 trally each taenia turns lateralward over the restiform body and forms the caudal 

 boundary of the corresponding lateral recess. 



THE MESENCEPHALON 



The midbrain or mesencephalon occupies the notch in the tentorium and 

 connects the rhombencephalon, on the one side of that shelf-like process of 

 dura, with the prosencephalon on the other (Fig. 81). It consists of a. dorsal 

 part, the corpora quadrigemina, and a larger ventral portion, the cerebral pe- 

 duncles. It is tunneled by a canal of relatively small caliber, called the cerebral 

 aqueduct, which connects the third and fourth ventricles and is placed nearer 

 the dorsal than the ventral aspect of the midbrain (Fig. 84). 



The cerebral peduncles (pedunculi cerebri, crura cerebri), as seen on the 

 ventral aspect of the brain, diverge like a pair of legs from the rostral border of 

 the pons (Fig. 83). Just before they disappear from view by entering the ven- 

 tral surface of the prosencephalon they enclose between them parts of the hypo- 

 thalamus, and are encircled by the optic tracts. On section, each peduncle is 

 seen to be composed of a dorsal part, the tegmentum, and a ventral part, the 

 basis pedunculi. Between the basis pedunculi and the tegmentum there inter- 

 venes a strip of darker color, the substantia nigra (Fig. 113). By dissection it is 

 easy to show that the basis pedunculi is composed of longitudinally coursing 

 fibers which can be traced rostrally to the internal capsule (Fig. 88). In the 

 other direction some of these fibers can be followed into the corresponding pyra- 

 mid of the medulla oblongata. On the surface two longitudinal sulci mark the 

 plane of separation between the tegmentum and the basis pedunculi. The 



