THE NERVOUS SYSTEM 1459 



pontine, and cervical. The eephalic flexure appears between the prosen- 

 cephalon and mesencephalon, and by it the prosencephalon is bent ventral- 

 ward; round the cephalic end of the notochord. on to its ventral aspect. 

 By reason of this flexure the prosencephalon is lowered, and the mesencephalon 

 occupies the summit of the encephalon. The cephalic flexure coincides with 

 the head-fold of the embryo, and the concavity of the flexure is directed 

 ventralwards. The pontine flexure, the concavity of which is directed dorsal- 

 wards, appears on the ventral waU of the metencephalon, or cephalic sub- 

 division of the hind-brain, in the region of the future pons Varolii. The 

 cervical flexure appears between the myelencephalon, or caudal subdivision 

 of the hind-brain, and the commencement of the spinal cord. Its concavity 

 is directed ventralwards, like that of the cephalic flexure. 



Secondary Cerebral Vesicles. — The middle primary cerebral vesicle remains 

 undivided, but the anterior and posterior primary cerebral vesicles are each 

 subdivided into two parts by constrictions which appear on their lateral 

 walls. The subdivisions of the anterior primary cerebral vesicle are known as 

 the first and second secondary cerebral vesicles, the former being the anterior 

 of the two. The middle primary cerebral vesicle, as stated, remains undivided, 

 and forms now, in series, the third secondary cerebral vesicle. The subdivisions 

 of the posterior primary cerebral vesicle are known as the fourth and fifth second- 

 ary cerebral vesicles. There are thus five secondary cerebral vesicles, named 

 first, second, third, fourth, 3x16^ fifth, of which the third is reaUy not secondary. 

 The first secondary cerebral vesicle represents the telencephalon (end-brain), 

 the second the diencephalon (inter-brain) or thalamencephalon (bedchamber- 

 brain), the third the mesencephalon (mid-brain), the fourth the metencephalon 

 (hind-brain), and the fifth the myelencephalon (marrow- or after-brain). 



Secondary Cerebral Vesicles. 

 First ^, Telencephalon (end-brain). 



Second /Diencephalon (inter-brain) ; or 



•'■ \ Thalamencephalon (bedchamber-brain). 



Third ,, Mesencephalon (mid-brain). 



Fourth .. Metencephalon (hind-brain). 



Fifth .. Myelencephalon (marrow- or after-brain). 



The constrictions, which subdivide the anterior primary cerebra vesicle 

 into telencephalon and diencephalon, are anterior to the origins of the optic 

 vesicles, and consequently these vesicles pertain to the diencephalon. 



In a general way. the telencephalon gives origin to the cerebral hemispheres 

 and the olfactory lobes. The diencephalon gives origin to the optic stalks 

 and the structures around the third ventricle — e.g., the optic thalaml, pineal 

 body, and posterior lobe of the pituitary body. The mesencephalon gives origin 

 to the structures around the aqueduct of Sylvius — namely, the crura cerebri 

 and corpora quadrigemina. The metencephalon gives origin to the cerebellum, 

 pons Varolii, and pontine part of the fourth ventricle. The myelencephalon 

 gives origin to the medulla oblongata and bulbar part of the fourth ventricle. 



The following table represents the formations just referred to: 



fTelencephal on = First secon- 



\ Diencephalon J vesicle 



Middle primarv "\ f Mesencephalon, 1 i-v j j 1 



cerebral vesicle / = \ or mid-brain J Third secondary vesicle 



D ^ ■ ^ [Rhombencephalon.! (Metencephalon = Fourth secon- 



Posterior primary IT"^" ucui^cpixaauu, ■ j ^^^ dary vesicle 



cerebral vesicle / | hind-brain f 1 Myelencephalon = Fifth secon- 

 v J \ dary vesicle 



