GENERAL DEVELOPMENT OF THE BRAIN. 1061 



bined contribution of the telencephalon and diencephaion. During- the fifth week 

 the diencephaion expands into a relatively large irregular space (Fig. 913), whose 

 roof and floor are thin and whose lateral walls are thickened by the masses of the 

 developing thalami. The hypothalamic region becomes the most dependent 

 part of the fore-brain and gives rise to the structures that later occupy the inter- 

 peduncular space on the base of the brain. The roof of the diencephaion remains 

 thin, does not produce nervous tissue and, in conjunction with the ingrowth of the 

 vascular pia mater, forms the velum interpositum and its choroid plexuses. The 

 pineal body.and the posterior lobe of the pituitary body arise as outgrowths from the 

 roof and floor of the diencephaion respectively. 



The mid-brain, or mesencephalon, at first large and conspicuous on account 

 of its elongation and prominent position at the summit of the brain-tube, does not 

 keep pace with the adjoining vesicles, and in the fully formed brain is represented by 

 the parts surrounding the aqueduct of Sylvius. Neither does it subdivide, but, while 

 its entire wall is converted into nervous tissue, retains its primary simplicity to a 

 greater degree than any of the other brain-segments. The lateral and ventral walls 

 of the mid-brain contribute the cerebral peduncles' ; its roof gives rise to the corpora 

 quadrigemina ; and its cavity persists as the narrow canal, the aqueduct of Sylvius, 

 that connects the third and fourth ventricles. 



The posterior vesicle, the hind-brain, or rhombencephalon, the largest of 

 the primary brain-segments, is the seat of striking changes. These include thicken- 

 ing and sharp forward flexion of the ventro-lateral walls, in consequence of which the 

 floor of the space becomes broadened out opposite the bend and assumes a lozenge- 

 shaped outline. The hind-brain is conventionally subdivided (Fig. 913) into a 

 superior part, the metencephalon, and an inferior part, the myelencephalon. 

 Its cavity, common to both subdivisions, persists as the fourth ventricle. 



The extreme upper part of the metencephalon, where it joins the mid-brain, 

 early exhibits a constriction, which by His has been termed the isthmus rhom- 

 bencephali and regarded as a distinct division of the brain-tube. In the fully formed 

 brain, the isthmus corresponds to the uppermost part of the fourth ventricle, just below 

 the Sylvian aqueduct, roofed in by the superior medullary velum that stretches 

 between the superior cerebellar peduncles. The thickened and markedly bent ventro- 

 lateral wall of the metencephalon gives rise to the pons Varolii, whilst in the roof of 

 the ventricle appears a new mass of nervous tissue, the cerebellum. 



The myelencephalon, soon limited below by the cervical flexure, shares in the 

 ventral thickening seen in the preceding division. Its floor and particularly its sides, 

 the latter at the same time spreading apart, form the medulla oblongata, which 

 below gradually tapers into the spinal cord. Its roof, in which thinness is always 

 a prominent feature, becomes more attenuated as development proceeds and is 

 converted into the inferior medullary velum and the tela chorioidea that close in this 

 part of the fourth ventricle. The subsequent invagination of this membranous 

 portion of the ventricular roof by the pia mater brings about the production of a 

 choroid plexus similar to that seen in the roof of the third ventricle. 



From the foregoing sketch of the changes affecting the embryonic brain-tube, it 

 is evident that the anterior and posterior primary vesicles undergo subdivision, while 

 the mid-brain remains undivided, five secondary brain-vesicles the telencepha- 

 lon, the diencephaion, the mesencephalon, the metencephalon and the myelencepha- 

 lon replacing the three primary ones. 



In consequence of the unequal growth of various parts of the cephalic segment 

 of the neural tube, the latter becomes bent in the sagittal plane at certain points, 

 so that, when viewed from the side, the axis of the developing human brain 

 describes an S-like curve (Fig. 912). These flexures, to which incidental reference 

 has been made, bring about a disturbance, for the most part temporary, in the 

 relations of the brain-segments, which in the lower vertebrates follow in regular order 

 along an axis practically straight. In the developing human brain, in which they 

 are most conspicuous, there are three flexures the cephalic, cervical, and pontile. 



The first of these, the cephalic flexure which appears towards the end of 

 the second week and before the neural tube has completely closed, is primary and 

 involves the entire head. It takes place in the region of the mid-brain and lies 



