CHAPTEK IX 



MID-BRAIN AND THALAMENCEPHALON 



CONTENTS. 1. General structure of the mesencephalon. 2. The thalamen- 

 cephalon. 3. Effects of total extirpation of fore-, inter-, and mid-brain in fishes ; 

 4. In amphibia ; 5. In birds ; 6. In mammals. 7. Effects of stimulating the 

 mesencephalon. 8. Effects of extirpating the corpora quadrigemina alone. 



9. Effects of dividing the whole or half the brain-stem at level of the mid-brain. 



10. Effects of incomplete or total removal of optic thalami. Bibliography. 



I. THE Mid-brain (mesencephalon) arises from the median primary 

 vesicle of the embryonic brain, which is interposed between the 

 hind -brain (pons and cerebellum) and the inter -brain (optic 

 thalamus). Of the cerebral vesicles this is the one that under- 

 goes least alteration during development. The changes consist 

 principally in a simple thickening of its walls and subsequent 

 restriction of the cavity, which is transformed into the aqueduct 

 of Sylvius. In the lower vertebrates it attains a more or less con- 

 spicuous development ; but in mammals its comparatively pre- 

 cocious development is arrested very early, and in man it develops 

 least of the five original parts of the brain. 



The mid-brain is usually divided into two parts : one ventral 

 the cerebral peduncles ; the other dorsal comprising the corpora 

 quadrigemina which in lower vertebrates are also known as the 

 optic lobes. 



The ventral portion of the mid-brain is divided into two parts 

 by the substantia nigra of Sommerung, the ventral of which is 

 termed the pes or crusta of the peduncle, the dorsal the tegmentum 

 (Figs. 242, 243). 



The first is the continuation of the pyramidal fibres of the pons 

 and bulb, with the addition of other longitudinal fibres which 

 come from the fore-brain ; the second is the continuation of the 

 formatio reticularis, with the addition of much grey matter and of 

 white ' fibres, some of which represent the continuation of the 

 superior cerebellar peduncles. The crura of the peduncles are 

 separated from one another ; the two tegmenta, on the contrary, 

 are united in the median plane along the raphe, and extend 

 dorsally on the side of the aqueduct into the corpora quadrigemina. 



486 



