1466 A MANUAL, OF ANATOMY 



the future temporal lobe, in a crescentic manner, so as to embrace the stalk 

 of the cerebral hemisphere. After the ependymal infolding has taken place, 

 vascular mesenchyme dips in between its two folds, and so a plica choroidea 

 is formed. From this choroidal fold the lateral choroid plexus of the cor- 

 responding side is formed. This plexus projects into the lateral ventricle, 

 but is excluded from the ventricular cavity by the ependyma of the wall, 

 previously infolded. When the lateral choroid plexus is withdrawn, the thin 

 ependymal covering of the plexus comes away with it, or is broken down. 

 Under these circumstances the choroidal fissure is really a fissure, inasmuch 

 as the lateral ventricle now opens upon the surface through it. The internal 

 prominence produced by the choroidal fissure is the lateral choroid plexus, 

 covered by ependyma, this prominence being very conspicuous. 



The other complete fissures will be found described in connection with the 

 cerebral hemispheres. 



The incomplete fissures are very numerous. The first two to make their 

 appearance are the calloso-marginal fissure, or sulcus cinguli, on the mesial 

 surface of the cerebral hemisphere, and the central sulcus or fissure of Rolando 

 on the external surface. These, along with the other incomplete fissures, will 

 be found described in connection with the cerebral hemispheres. 



The cavity of the vesicle of the cerebral hemisphere forms on either side 

 the lateral ventricle, which is very much curtailed by the thickening under- 

 gone by the vesicular walls, and the internal prominences produced by the 

 complete fissures. As the frontal lobe undergoes development the body of the 

 ventricle extends forwards into it, and so the anterior cornu of the ventricle 

 is formed. As the temporal lobe grows in a downward and forward direction, 

 it carries with it an extension of the body of the ventricle, this extension 

 constituting the inferior or descending cornu. As the occipital lobe becomes 

 developed at a later period the body of the ventricle extends backwards into 

 it, and so the posterior cornu is formed. Meanwhile the foramen of Monro 

 on either side, originally large, is being gradually much diminished in size. 



Basal Ganglia. — ^The basal gangha of each cerebral hemisphere are as 

 follows : 



Corpus striatum. 

 Claustrum. 

 Amygdaloid nucleus. 



They are all developed from the deep part of the much thickened cortical 

 substance which forms the floor of the Sylvian fossa. The claustrum and 

 amygdaloid nucleus remain of small size, but the nucleus caudatus of the 

 corpus striatum forms a conspicuous prominence in the lateral ventricle, as 

 it bulges into the anterior cornu of that cavity. 



Commissures. — The commissures are as follows: 



1. Corpus callosum. 4. Anterior. 



2. Fornix. 5. Posterior. 



3. Hippocampal. 6. Middle. 



7. Habenular. 



At an early period in the development of the cerebral hemispheres the 

 interhemispherical (great longitudinal) fissure leads directly dow n to the roof 

 of the diencephalon. At a later period the roof of the diencephalon is 

 separated from the great longitudinal fissure by two commissures, placed one 

 above the other. The upper commissure is the corpus cedlosum, and the lower 

 one is the fornix. 



The corpus callosum and fornix are formed in connection with the lamina 

 terminalis. This lamina, which is originally thin, becomes thickened, and 

 the mesial walls of the hemispheres come together and fuse directly over 

 and anterior to it. At the same time the united walls of the hemispheres 

 blend with the upper margin of the lamina terminalis. The area of fusion 

 of the walls of the hemispheres now extends both anteriorly and posteriorly, 

 and this area forms a basis or plate for the formation of the corpus callosum 

 and fornix. Transverse fibres are developed over the dorsal aspect of the 

 area, which cross from the neopallium of one hemisphere to that of the other. 



