178 



TOPOGRAPHICAL ANATOMY OF THE 



part of the neural tube is expanded, and, early in embryonic lite, is marked oflf by 

 two constrictions into the i\\reG primary cerebral vesicles — the hind-brain or rhomben- 

 cephalon, the mid-brain or mesencephalon, and the fore-brain or prosencephalon. 

 Certain Hexures also occur in this ])art of the neural tube. The first to appear is the 

 cephalic Jiexnre, in the region of the mid-brain, produced by the fore-brain and the 

 anterior part of the head being bent round the anterior end of the foregut and the 

 notochord. A forward bending of the whole head leads to the formation of the 

 cervical flexure, which marks the junction of the hind-brain with the spinal cord. 

 This flexure is indifferently marked in the embryos of domestic mammals, and does 

 not persist after birth. A third bend, with its convexity in a direction opposite to 

 that of the other two, namely, downwards instead of upwards, forms in the region of 

 the future jjons, and is therefore known as the pontine flexure. 



Tlie rhombencephalon or hind-brain is the largest of the three primary cerebral 

 vesicles, and is usually held to be divisible into two parts — metencephalon and 

 rayelencephalon. From the myelencephalon — the jiosterior segment — the medulla 



Olfactory bulb. 



Telencephalon 



Diencephalon, 

 Mesencephalon, 

 Metencephalon._ 



Myelencephalon. 



Medulla spinalis. _.. 



•Foramen interventriculare. 

 Ventriculus lateralis. 

 Ventriculus tertius. 



Aquseductus cerebri. 

 1 Ventriculus quartus. 



Canalis centralis. 



Fig. 82. — Diagram of the ventricular system of the brain. 



oblongata is formed, largely by a thickening of its lateral walls, but also by the 

 development of decussating nerve-fibres in the floor plate. The roof plate, from a 

 falling apart of the lateral walls, becomes greatly expanded, remains chiefly 

 ependymal, and forms the thin posterior part of the roof of the fourth ventricle. 

 From the metencephalon are developed the pons and cerebellum. 



That part of the hind-brain which joins the mid-brain is narrower than the 

 rest, and to it the name of isthmus rhombencephali is given. 



The mesencephalon or mid-brain remains undivided, and, though comparatively 

 large in the early embryo, forms only a small part of the fully developed brain. 

 From the circumstance that nerve tissue is developed in every part of the wall of 

 this vesicle, its cavity — the aqueeductus cerebri — is very restricted in comparison 

 wdth the ventricles that represent the cavities of the other cerebral vesicles. The 

 dorsal (alar) zone of the mid-brain becomes the corpora quadrigemina, while the 

 ventral (basal) zone develops into the cerebral peduncles. 



The prosencephalon or fore-brain at an early stage is particularly wide opposite 

 lateral diverticula— the optic wesicZes— which are responsible for the formation of the 

 retinae and the optic nerves. Changes in the form of the fore-brain are numerous 

 and complicated, but it may be stated briefly that they result in a division of this 

 part of the embryonic brain into an anterior telencephalon and a posterior 



