FORNIX. 377 



tremitj, and the two ends carefully dissected away. The anterior portion 

 will be retained only by the septum lucidum, but the posterior will be 

 found incorporated with the white layer beneath, which is the fornix. 



FORNIX. The fornix (arch) is a triangular lamina of white substance, 

 broad behind, and extending into each lateral ventricle : narrow in front, 

 where it terminates in two crura, which arch downwards to the base of 

 the brain. The two crura descend in a curved direction to the base of 

 the brain, embedded in grey substance, in the lateral walls of the third 

 ventricle, and lying directly behind the anterior commissure. At the base 

 of the brain they make a sudden curve upon themselves and constitute the 

 corpora albicantia, from which they may be traced upwards to their origin 

 in the thalami optici. Opening transversely beneath these two crura, just 

 as they are about to arch downwards, is the foramen of communication 

 between the lateral and the third ventricles, the foramen of Monro ; or 

 foramen commute anterius. The choroid plexuses communicate, and the 

 veins of the corpora striata pass through this opening. 



The lateral thin edges of the fornix are continuous posteriorly with the 

 concave border of the hippocampus major at each side, and form the nar- 

 row white band called corpus fimbriatum (posterior crus of the fornix). 

 In the middle line the fornix is continuous with the corpus callosum, and 

 at each side with the hippocampus major and minor. Upon the under 

 surface of the fornix towards its posterior part, some transverse lines are 

 seen passing between the diverging corpora fimbriata: this appearance is 

 termed the lyra (corpus .psalloides), from a fancied resemblance to the 

 strings of a harp. 



The fornix may now be removed by dividing it across anteriorly, and 

 turning it backwards, at the same time separating its lateral connexions 

 with the hippocampi. If the 



student examine its under sur- Fig. 171.* 



face, he will perceive the lyra 

 above described. 



Beneath the fornix is the 

 velum interpositum, a duplica- 

 ture of pia mater introduced 

 into the interior of the brain, 

 through the transverse fissure. 

 The velum is continuous at 

 each side with the choroid 

 plexus, and contains in its in- 

 ferior layer two large veins 

 (the vena Galeni) which re- 



* The mesial surface of a longitudinal section of the brain. The incision has been 

 carried along the middle line; between the two hemispheres of the cerebrum, and 

 through the middle of the cerebellum and medulla oblongata. 1. The inner surface of 

 the left hemisphere. 2. The divided surface of the cerebellum, showing the arbor vitae. 

 3. The medulla oblongata. 4. The corpus callosum curving downwards in front to ter- 

 minate at the base of the brain, and rounded behind to become continuous with 5, the 

 fornix. 6. One of the crura of the fornix descending to 7, one of the corpora albicantm. 



The septum lucidum. 9. The velum interpositum, communicating with the pia mater 

 of the convolutions through the fissure of Bichit. 10. Section of the middle commissure 

 situated in the third ventricle. 11. Section of the anterior commissure. 12. Section ol 

 the posterior commissure; the commissure is somewhat above and to the left of the 

 numeral. The interspace between 10 and 11 is the foramen commune anterius, in 

 which the crus of the fornix (G) is situated. The interspace between 10 and 12 is th 



32* 



