568 Dynamic Theory. 



edge of this lamina is attached to the optic chiasm, which is under the 

 front end of the ventricle and just in advance of the tuber cinereum. 

 Above and in advance of the opening into the tuber cinereum, is the 

 foramen of munro, which is the anterior exit of the third ventricle. The 

 foramen of munro is bounded in front by the anterior pillars of the f or- 

 nix, which arch over it toward the rear, and the openings to the right 

 and left under the edges lead from the foramen to the right and left 

 lateral ventricles. It is Y-shaped, the two prongs connecting above and 

 to the right and left with the lateral ventricles, while the stem descends 

 to connect with the third ventricle. The lateral ventricles, which lie 

 above and to the right and left of the third ventricle, have been 

 described, and mention has been made of the olfactory ventricle which 

 exists in the olfactory bulb in most vertebrates below man, with a canal 

 leading to it from the advanced point of the anterior cornu of the lat- 

 eral ventricle. 



There is one more ventricle to be found in man and monkey, but not 

 in other vertebrates, which is called the fifth ventricle. This is formed 

 in the wall of the septum lucidum by the separation of its two lamina 

 or leaves. It does not connect with the other ventricles, and is sup- 

 posed by Quain to be derived from the great longitudinal fissure. 



Thus this remarkable series of cavities extends from end to end of the 

 cranio-spinal axis, communicating with each other throughout ( except 

 the fifth ). They are also all lined with the inner membrane of the 

 arachnoid, which, extending through the spinal canal, follows the walls 

 of all the cavities, and finally passes out of the interior of the brain 

 through an opening under the posterior end of the corpus callosum, to- 

 gether with the pia mater, and spreads itself over the external surface 

 of the brain, and the cranial nerves as they leave the cranial cavity. 



FIG. 277. Section of convolutions of Posterior 

 lobe of Cerebrum, showing- gray cortex and white 

 band of fibrous nerve matter peculiar to tins lobe. 



( Owen.) 



Structure of the Cerebrum. The cere- 

 brum and cerebellum are alike in the fact 

 that the gray substance is on the outside* 

 arranged in a coat over the surface, while 

 the interior portions of their substance 

 consist of white fibrous matter. This 

 white matter consists of nerve tubules 

 FIG. 277. ~ m juxtaposition, and almost infinite in 



number. They have been described. They are in two series, one of 

 which may be called the converging fibres, since, like the spokes of a 

 wheel, they converge from the periphery, or cortex, toward the optic 

 thalami and internal capsuJes. The other fibres are the commissural. 



