598 DEVELOPMENT OF FCETAL BRAIN. 



mammillares, and all the remaining fibres which are still very 

 numerous, pass below the thalamus, and after sending up a band 

 into the corpus striatum, radiate like the branches of a fan into 

 the membranous hemisphere. These radiating fibres having gain- 

 ed the top, redescend in the middle line to reach the crura of the 

 fornix ; some of the fibres of both hemispheres uniting at the 

 front to form the corpus callosflm. 



The fibres described as descending to the eminentia mammil- 

 lares from the crura cerebri, mount upwards as the pillars of the 

 fornix, and are continued first backwards and then downwards 

 as the posterior pillars of the same body, to the bottom of what 

 is to become the middle cornua of the lateral ventricles. A sin- 

 gular and interesting connection is thus early seen in the 

 course of the fibres, which indicates the shape of the future 

 ventricles, and shows at the same time that the development of 

 the brain takes place in two directions. 



In the foetus Q^ five months, we find the medulla spinalis 

 still tubular and capable of being inflated in a downward 

 direction from the fourth ventricle. 



The tube is expanded at the two enlargements of the 

 medulla, where the brachial and sacral plexuses of nerves have 

 their connection with it. 



No corpus olivare is yet developed. The cerebellum now 

 shows its divisions into central or fundamental, and lateral 

 portions, the latter of which cover over behind the bottom of 

 the fourth ventricle. The transverse fibres of the pons are 

 better developed, and the ascending stratum of fibres called the 

 valve of Vieussens, sent up by the pons to the tubercula quadri- 

 gemina, is well seen. The two hemispheres of the cerebrum 

 are prolonged more backward, and are about an inch and a 

 quarter in length. As yet they exhibit no convolution, except 

 at their internal and anterior part. 



The masses of the tubercula quadrigemina do not yet present 

 the eminences called nates and testes. 



The corpus callosum and the fornix have increased in size, 

 and two thin lamina extend from the latter to the former, 

 forming the commencement of the septum lucidum. These 

 lamina have a ventricle between them, the fifth, that commu- 



