352 THE MID BR A IK. 



later period, and are hardly developed in the Monotremata and Mar- 

 supialia. 



The cerebellum is connected with the roof of the mid-brain in front and 

 with the choroid plexus of the fourth ventricle behind by delicate mem- 

 branoiis structures, known as the velum medullae anterius (valve of 

 Vieussens) and the velum medullae posterius. 



The pons Varolii is formed on the ventral side of the floor of the 

 cerebellar region as a bundle of transverse fibres at about the same time as 

 the olivary bodies. 



The mid-brain. The changes undergone by the mid-brain are 

 simpler than those of any other part of the brain. We have already 

 seen that the mid-brain, on the appearance of the cranial flexure, 

 forms an unpaired vesicle with a vaulted roof and curved floor, at 

 the front end of the long axis of the body (fig. 118, MB). It is at 

 this period in most Vertebrates relatively much larger than in the 

 adult ; and it is only in the Teleostei that it more or less retains in 

 the adult its embryonic proportions. 



The cavity of the mid-brain, greatly reduced in size in the higher 

 forms, is known as the iter a tertio ad quartum ventriculum, or aque- 

 ductus Sylvii. 



The roof of the mid-brain is sharply constricted off from the 

 divisions of the brain in front of and behind it, but these constrictions 

 do not extend to the floor. 



In some Vertebrates the region of the mid -brain is stated to 

 undergo hardly any further development. In the Axolotl it remains 

 according to Stieda^ as a simple tube with nearly uniformly thick 

 walls. In the majority of forms it undergoes, however, a more com- 

 plicated development. 



In Elasmobranchs the sides become thickened to form the optic lobes, 

 which are soon separated by a median longitudinal groove. The floor be- 

 comes thickened to form the crura cerebri. The primitive simple median 

 cavity becomes imperfectly divided into a median portion below, and two 

 lateral diverticula in the optic lobes. 



In Teleostei the changes, residting in the formation of (1) a pair of 

 longitudinal ridges projecting from the roof into the cavity of the iter, 

 constituting the fornix of Gottsche, and (2) of the two swellings on the 

 floor, forming the tori semicirculares, are more complicated, but have not 

 been satisfactorily worked out. In Bombinator and the Anura generally 

 the changes are of the same nature as those in Elasmobranchii, except that 

 the prolongations of the ventricle into the optic lobes are still further con- 

 stricted ofl" from the median portion, which forms the true iter. 



In Reptilia and Aves the development of the mid-brain takes place on 

 the same type as in Elasmobranchii and the Anura. In Birds the optic 

 lobes are pushed very much aside, and the roof of the iter is greatly thinned 

 out. In Mammalia the sides of the mid-brain give rise to two pairs of 

 prominences — the corpora quadrigemina — instead of the two optic lobes of 



^ "Ueb. d. Bau d. centralen Nervensystem d. Axolotl." Zeit. f. wiss. Zooh, Vol. 

 XXV. 1875. 



