THE NERVOUS SYSTEM 547 



Commissures. The morphological changes which the brain under- 

 goes are accompanied by the development of fiber tracts which connect 

 the various parts of the brain and effect its functional unity. Among 

 these, none are more important than the commissures which unite the 

 lateral halves of the brain. These are formed by bundles of neurites 

 which grow from neuroblast cells in each side of the brain to corresponding 

 nuclei in the other side. The anterior and hippocampal commissures 

 develop in the lamina terminalis. Following the elongation of the 

 cerebral hemispheres, the hippocampal commissure is carried caudad 

 and incorporated in the posterior part of the fornix. The fibers of the 

 anterior commissure connect the olfactory lobes with the hippocampus of 

 the opposite side; those of the hippocampal commissure mostly connect 

 the hippocampus with the hypothalamus of the opposite side. (Fig. 425) 



In correlation with the growth of the cerebral cortex, the corpus 

 callosum develops just above and anterior to the hippocampal commissure. 

 When it first appears, the corpus callosum is semicircular in cross section; 

 but as development proceeds, it assumes, in a five months embryo, the 

 elongated sickle-shape characteristic of the adult. Between the fornix 

 and corpus callosum, the thin median walls of the hemispheres are stretched 

 to form the median septum pellucidum. Between the two layers of this 

 septum there frequently develops a cavity erroneously known as the 

 fifth ventricle. (Fig. 383) 



Chorioid Plexus. From the roof of the forebrain, in the region where 

 telencephalon and diencephalon merge into one another, a chorioid plexus, 

 which involves numerous blood vessels as well as brain wall, grows down 

 into the cavities of the brain. This plexus penetrates the paired ven- 

 tricles of the hemispheres and the median unpaired third ventricle of the 

 diencephalon. 



Diencephalon. The wall of the posterior half of the primary fore- 

 brain differentiates four zones, roof plate, epithalamus, thalamus, and 

 hypothalamus. The roof-plate forms a chorioid plexus, which is con- 

 tinuous with that of the telencephalon, and which pushes into the ven- 

 tricle. An outgrowth from the epithalamus becomes the pineal gland. 

 A sulcus divides the thickened lateral wall into thalamus and hypo- 

 thalamus. The thalamus becomes a massive thickening and unites with 

 the corpus striatum. 



From the hypothalamus develop the tuber cinereum, mammillary 

 bodies, infundibulum, and the neural lobe of the pituitary gland. The 

 tuber cinereum and mammillary bodies, thickenings of the posterior wall 

 of the infundibulum, become connected with the hippocampus by means 

 of a fiber tract, and are therefore believed to have an olfactory function. 

 The infundibulum is formed as a funnel-shaped pit with apex towards the 



