THE BRAIN. 175 



The first bend is the most extensive, the axis of the vesicles 

 being bent to a right angle. These bends have been and 

 will be disregarded in the descriptions, as they would serve 

 to confuse the reader. 



The changes in the interbrain : The development of the 

 optic thalami has been given above. The floor of the 

 vesicle drops downward to form a blind tube the infundi- 

 bulum ; the end of the infundibulum becomes enlarged, and 

 meets an upward pouch from the pharynx of the foetus. 

 This pharyngeal pouch becomes narrowed at its neck and 

 finally cut off from the pharyngeal cavity and remains at- 

 tached to the infundibulum. The rounded mass thus formed 

 from the brain vesicle and the primitive pharynx is the 

 hypophysis or the pituitary body. The cerebral portion 

 constitutes a small central mass ensconced within the 

 horseshoe shaped mass from the pharynx. Fig. 20. 



The elevation (depression, really) at the junction of the 

 infundibulum and floor of the vesicle is the tuber cinereum. 



On each side of the median line two elevations appear, 

 which are the corpora albicantia. Forward from the tuber 

 cinereum the lamina terminalis is continuous under the 

 name of the lamina cinerea. 



The midbrain : The floor becomes thickened by the 

 downward extension of fibres from the greatly enlarged 

 forebrain (cerebrum), and these masses of fibres are called 

 the crura cerebri. The roof of the midbrain becomes 

 bunched to form the corpora quadrigemina. Its central 

 canal is the aqueduct of Sylvius. 



The hindbrain : It expands upward, outward, and back- 

 ward until the result is the cerebellum, while its thicken- 

 ing in front and below by numerous transverse fibres forms 

 the pons. 



The afterbrain : In like manner forms the medulla, by 



