DEVELOPMENT OF THE BRAIX. 



335 



fore-brain to form the third ventricle. These two ventricles and the 

 aqueduct are continuous with the central canal of the spinal cord and 

 represent the original cavity of the medullary tube. 



In an embryo of 10 mm. (Fig. 390, B) the hind-brain may be subdi- 

 vided into the myelence phalon posteriorly and the metencephalon anteriorly. 

 The constriction between the hind-brain and mid-brain is called the 

 isthmus. The mesencephalon remains undivided; the fore-brain is repre- 

 sented by the diencephalon posteriorly and the telencephal&n anteriorly. 

 Thus there are six funda- 

 mental subdivisions of the 

 brain. Their further de- 

 velopment is illustrated in 

 the median sagittal sections 

 of the brain, Figs. 392 and 

 393, and may be briefly de- 

 scribed as follows. 



The myelencephalon 

 becomes the medulla oblon- 

 gata. It transmits the 

 fibers passing between the 

 cord and the brain; it re- 

 ceives the sensory roots of 

 the vagus and glossopharyn- 

 geal nerves and contains 

 the groups of cell bodies 

 from which their lateral 

 roots arise [the lateral root 

 of the vagus being called 

 the accessory nerve]. It 

 also contains the cell bodies 

 from which arise the ven- 

 tral roots which make the 



hypoglossal nerve. (These nerves are shown in Fig. 113, p. 96, and in 

 Fig. 391, B.) The medulla also includes groups of cell bodies, the pro- 

 cesses of which do not leave the central nervous system. Such groups 

 are called nuclei; the gray substance in most of the ventral portion of 

 the brain is in the form of separate nuclei and not in continuous columns 

 as in the cord. 



The metencephalon produces the^pons ventrally and the cerebellum 

 dorsally. The pons receives the sensory roots of the trigeminal, inter- 

 mediate and acoustic nerves; it gives rise to the lateral roots of the tri- 



sp.c. 



FIG. 390. A, THE BRAIN OF A 4.0 MM. HUMAN EMBRYO 

 (after Bremer); B, THE BRAIN OP A 10.2 MM. EMBRYO 

 (after His). 



Except the isthmus, is., the principal subdivisions of the brain 

 are indicated by prefixes of the term encephalon; sp. c., 

 spinal cord; h., hemisphere; o. v., optic vesicle; r., rhinen- 

 cephalon ; v., roof of the fourth ventricle. 



