THE NATUBE OF THE BKAIN. 



515 



CEPHALIC 

 FLEXURE 



of the flexure. If this wall is strong and resisting, like the floor-plate of the 

 neural tube (in the cases of the cephalic and cervical flexures) the bending does 

 not affect the outline of the tube (in section) very materially. But when the 

 strain is thrown upon the thin roof-plate during the development of the pontine 

 flexure it is not strong enough to resist; it becomes stretched and allows the 

 side walls of the neural tube to splay laterally in precisely the same manner as 

 occurs when a rubber tube is bent towards a side which has been split (or 

 weakened) longitudinally (Fig. 457). This mechanical factor determines the form 

 assumed by the hind-brain at the end of the first month ; and gives its cavity, 

 the fourth ventricle, a lozenge or rhomboid form, when seen from its dorsal aspect 

 through the thin translucent roof. For this reason the hind-brain is known as 

 the rhombencephalon. 



The rhombencephalon forms at first more than half of the encephalon, and as 

 it expands it appears to become marked off from the rest by a constriction (the 

 isthmus rhombencephali). 



The development of the pontine 

 flexure subdivides the rhomben- 

 3ephalon into two parts, one joined 

 to the spinal medulla, the myelen- 

 3ephalon, and the other, joined to the 

 :est of the brain, the metencephalon. 



In the myelencephalon develop 

 jhe nuclei of the nerves that regulate 

 ihe activities of the heart, lungs, and 

 i considerable part of the alimentary 

 3anal, and also the receptive nuclei 

 )f the nerves of taste. It is known 

 is the medulla oblongata. 



The insertion of the nervus 

 r icusticus in the neighbourhood of 

 ;he outsplayed lateral angle of the 

 hombencephalon leads to the pro- 

 found transformation of the meten- 

 iephalon. The nervus acusticus 

 conveys into the hind-brain impulses 

 /vhich are stimulated by movements 

 )f fluid in the closed sac developed 

 rom the otic vesicle (Fig. 443, p. 501). 

 Che truly acoustic function of this 

 ipparatus is called into activity 

 vhen the movements of this fluid 

 ire caused by waves of sound transmitted to it from the outside world. But 

 t is obvious that motion may also be set up in this fluid by changes in position 

 >f the body itself; in other words, movements in the fluid of the otic vesicle 

 nay stimulate nerves to convey to the brain information concerning the position 

 md movements of the body itself. A great mass of nerve-cells develops around 

 he insertion of the nervus acusticus (that part of it, however, which is called 

 r estibular and is not concerned with the function of hearing) to make use of 

 his information for the regulation of the movements of the body in balancing 

 >r equilibration. To enable this terminal vestibular nucleus the better to 

 >erform this function of equilibration, depending as it does upon the co-operation 

 nd adjustment of the movements of vast numbers of widely separated muscles, 

 lerye tracts coming from muscles and skin areas of all parts of the body make 

 heir way into this vestibular nucleus; and it expands and forms a great 

 xcrescence which is known as the cerebellum. And as this cerebellum has to 

 djust the activities of all the muscles of the body it necessarily becomes the 

 - 'reat organ of muscular co-ordination, and as such it is made use of by those 

 s of the brain which have to initiate and control complex actions such as 

 killed movements. It will be shown in the subsequent account how the 



34 a 



FIG. 457. PROFILE VIEW OF THE BRAIN OF A HUMAN 

 EMBRYO OF TEN WEEKS (His). 



The various cerebral nerves are indicated by numerals. 



A, Cerebral diverticulum of hypophysis cerebri. 

 B, Buccal diverticulum of hypophysis cerebri. 



