C80 THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



posterior opening (foramen commune posterius^) ; in front, into the common 

 anterior opening (foramen commune anterius) : orifices which will be described 

 with the interior of the isthmus. 



Outwardly, the thalamus optici shows two prominences called the corpora 

 geniculata, from which arise the second pair of nerves ; placed one before 

 the other, the posterior nearer the middle line than the anterior, these two 

 projections are distinguished as external and internal. The corpus genicu- 

 iatum externum is always more voluminous, better defined, and situated on a 



Fig. 324. 



LATERAL VIEW OF THE ISTHMUS. 



1, Medulla oblongata ; 2, Corpus restiforme ; 3, Lateral fasciculi of the medulla 

 oblongata ; 4, Inferior fasciculus, or corpus pyramidale ; 5, Pons Varolii ; 6, 

 Middle peduncle of the cerebellum ; 7, Cerebral peduncle ; 8, Testis ; 9, Natis ; 

 10, Corpus geniculatum internum ; 11, Corpus geniculatura externum ; 12, Optic 

 nerve ; 13, Fourth nerve resting on the band of Reil ; 14, Sensitive root of the 

 trigeminal nerve; 15, Its motor root ; 16, Facial nerve; 17, Auditory nerve. 



more elevated plane than the internal corpus geniculatum, which is united to 

 the posterior corpora quadrigemina by an oblique band (Figs. 323, 10 ; 

 324, 11). 



Behind, the thalami optici appear to be notched to receive the nates, 

 which they slightly inclose. 



In front, they are separated from the corpus striatum by a groove, at the 

 bottom of which is a narrow strip named the semicircular band (tcenia semi' 

 circularis). 



Pineal Gland or Conarium. (Fig. 323, 14.) 



This name has been given to a small tubercle of a reddish-brown colour, 

 in the form of a pine-cone, enveloped by a duplicature of pia mater from 

 the velum inter positum, with its apex upwards, and its base resting on the 

 common posterior opening, which it closes, and around which it is attached 

 by a circular lamella. 



From this lamella is detached, in front, two fibrous cords the anterior 



