424 



THE HIND-BRAIN. 



The hind-brain. The hind-brain is at first an elongated, 

 funnel-shaped tube, the walls of which are of a nearly uniform 

 thickness, though the roof and floor are somewhat thinner than 

 the sides. It forms a direct continuation of the spinal cord, into 

 which it passes without any sharp line of demarcation. The 

 ventricle it contains is known as the fourth ventricle. 



The sides become in the chick marked by a series of transverse con- 

 strictions, dividing it into lobes, which are somewhat indefinite in number. 

 The first of these remains permanent, and its roof gives rise to the cerebellum. 

 It is uncertain whether the other constrictions have any morphological 

 significance. More or less similar constrictions are present in Teleostei. 

 In Elasmobranchii the medulla presents on its inner face at a late period a 

 series of lobes corresponding with the roots of the vagus and glossopharyngeal 

 nerves, and it is possible that the earlier constrictions may potentially 

 correspond to so many nerve-roots. 



Throughout the Vertebrata an anterior lobe of the hind- 

 brain becomes very early marked off, so that the primitive 

 hind-brain becomes divided into two regions which may be 



cc 



AOA 



FIG. 249. SECTION THROUGH THE HIND -BRAIN OF A CHICK AT THE END 

 OF THE THIRD DAY OF INCUBATION. 



IV. Fourth ventricle. The section shews the very thin roof and thicker sides of 

 the ventricle. Ch. Notochord ; CV. Anterior cardinal vein; CC. Involuted auditory 

 vesicle ; CC points to the end which will form the cochlear canal ; RL. Recessus 

 labyrinth! (remains of passage connecting the vesicle with the exterior) ; hy. Hypoblast 

 lining the alimentary canal; AO., AOA. Aorta, and aortic arch. 



