FffiTUS OF SIX MONTHS. 599 



nicates with the third which is now provided with its anterior 

 commissure and the infundibulum. >- 



The walls of the hemispheres are very thin ; on laying them 

 open, the eminence analogous to the corpus striatum is found, 

 surrounded and imbedded as it were, in the ascending fibres of 

 the crus cerebri. It is not yet connected to the thalamus by 

 the band called tenia striata. The olfactory nerves are seen 

 arising in front from a hollow bulb, which communicates on each 

 side with the anterior cornu of the lateral ventricle. The optic 

 nerves come off partly from the mass of the tubercula quadri- 

 gemina and from the thalami. The dura and pia mater are now 

 well seen. In this stage of development, the brain of the foetus 

 resembles much the perfect brain of the lower mammalia. At 

 the sixth month the arachnoid membrane becomes apparent. 

 The hemispheres of the cerebrum are still smooth on their upper 

 surface, but much thicker and have extended so far backwards 

 as to cover the tubercula quadrigemina and the cerebellum. 

 The corpus callosum is so far developed backward, that the two 

 hemispheres can no longer be separated in the middle line so as 

 to expose the great ventricular cavity, without lacerating some 

 of the fibres of that commissure. The fourth ventricle or the 

 ventricle of the cerebellum is diminished much in size, the pons 

 has greatly increased, and the valve of Vieussens is marked 

 with the grey substance, forming the bands of Wensel. The 

 optic thalami are not united by their middle commissure. But 

 the anterior commissure uniting the two corpora striata together 

 has become apparent. At the seventh month, the cavity in the 

 medulla spinalis is found much diminished, and the inner face of 

 its walls is covered with a granular substance, closely adherent 

 to the pia mater which enters in at the median fissures. In the 

 region of the back the medulla is very narrow, not more than a 

 line in diameter, while it is double that size at the points at which 

 it is connected with the roots of the brachial and lumbar plexu- 

 ses. At the upper part of the medulla oblongata it is found 

 about four lines and a half in diameter. 



The cortical substance of the cerebrum has not taken its 

 proper cineritious hue, a result which is only effected after birth. 

 On the olivary or lateral fasciculus of this portion now appears 



