756 



DEVELOPMENT OF THE NERVOUS SYSTEM. 



The three constituent parts of the medulla oblongata beg-in to ]je distrnguisheJ 

 about the thii-d month ; first the rr.stiform bodies, which are connected with the 

 commencing cerebellum, and afterwards the anterior pjTamids and olives. The 

 anterior puminidx become prominent on the surface and distinctly defined in the 

 fifth month ; and by this time also their decussation is evident. The oUranj 

 fasciculi are early distinguishable, but the proper olirary hochj. or tubercle, doe's 

 not appear till about the sixth month. The fam'wlai c'merea; of the fourth 

 ventricle can be seen at the fomth or fifth month, bub the nvhltc strue not until 

 after birth. • 



Cerebellum. — In the human embiyc the cerebellum exists at the end of the 

 second month, as a delicate medullary lamina, forming an arch behind the 

 corpora quadrigemina across the widely open primitive medullary tube. 



According to Bischofi", the cerebellum does not commence, as was previously 

 supposed, by two lateral plates which grow up and meet each other in the middle 

 line ; but a continuous deposit of nervous substance takes place across this part 



Fig. 556. Fig. 556. — Brain and Spinal Cord exposed feom behind i:v 



A FcETus OF THREE MONTHS (from Kolliker). 



h, the hemispbei-es ; in, the mesencephalic vesicle or corpora 

 qnadrigemina, c, the cerebellum ; below this are the medulhi 

 oblongata, mo, and fourth ventricle, with remains of the meni- 

 brana obturatoria. The spinal cord, s, extends to the lower end 

 of the sacral canal and presents the brachial and crural 

 enlargements. 



of the medullary tube, and closes it in at once. This layer 

 of nervous matter, which is soon connected with the 

 coi-pora restiformia, or inferior peduncles, increases gradu- 

 ally up to the fourth month, at which time there may be 

 seen on its under sui'face the commencing corpus th-ntatvm. 

 In the fifth month a division into five lobes has taken place ; 

 at the sixth, these lobes send out folia, which are at first 

 simple, but aftei-wards become subdivided. Moreover, the 

 hem i. '<])?! eres of the cerebellum are now relatively larger 

 than its median portion, or worm. In the seventh month 

 the organ is more complete, and thejfocoihis and 2Mt<terior 

 rein in, with the other parts of the mferior vennifoiin 

 process, are now distinguishable, except the amyrjdalo', 

 which are later in theii' appearance. 

 Of th.ej}eil-incles of the cerebellum, the inferior pair (corpora restiformia) are 

 the first seen — viz., about the thii-d month ; the middle peduncles are perceptible 

 in the foui-th month ; and at the fifth, the superior pedmicles and the Vieussenian 

 valve. The pons VfiroHi is formed, as it were, by the fibres from the hemispheres 

 of the cerebellvun embracing the pyramidal and olivary fasciculi of the meduUa 

 oblongata. According to Baer, the bend which takes place at this part of the 

 encephalon thrusts down a mass of nervous substance before any fibres can be 

 seen ; and m this substance transverse fibres, continuous with those of the 

 cerebellum, are afterwards developed. From its relation to the cerebellar hemi- 

 spheres the pons keeps pace with them in its gi'owth ; and. in conformity with 

 this relation, its transverse fibres are few, or entirely wanting, in those animals 

 in which there is a con-esponding deficiency or absence of the lateral parts of the 

 cerebellum. 



Parts connected with, the Mesencephalon. — The eorjioni qvndricfemina are 

 formed in the upper part of the middle cephalic vesicle ; the hollow in the 

 interior of A^diich communicates with those of the first and thhd vesicles. Tlie 

 coi-pora qnadrigemina, in the early condition of the human embryo, are of great 

 proportionate volume, in harmony with what is seen in the lower vertebrata ; 

 but subsequently they do not grow so fast as the anterior parts of the ence- 

 phalon, and are therefore soon overlaid by the cerebral hemispheres, which 



