THE SECOND AND THIRD CEREBRAL VEHICLES. 



67 



The third cerebral vesicle : mesencephalon : mid-brain. In this region 

 no expansion of the vesicle with thinning of the roof occurs, as in the others, but, on 

 the contrary, the roof undergoes considerable thickening (fig. 74). About the third 

 month, this thickening becomes separated into two by a median groove. These cor- 

 respond with the corpora bigemina of lower vertebrates ; it is only in mammals that 

 they become further subdivided by a transverse furrow. This appears in man about 

 the fifth month, and the eminences, which are at first large in proportion to the size 



Fig. 75. FffiTAL BRAIN OF THE 

 THIRD MONTH. (His.) 



The brain is represented in 

 profile, but the external vrall of 

 the right hemisphere has been 

 removed to show the interior of 

 the lateral ventricle with the cor- 

 pus striatura curving round the 

 bend of the fossa of Sylvius. The 

 curved projections above the cor- 

 pus striatum are infoldings of the 

 mesial wall of the hemisphere 

 vesicle. The lettering as in fig. 74. 



of the brain, thus become 

 the corpora quadrigemwa. 



The fibres of the third 

 nerve originate in the ven- 

 tral lamina of this part of 

 the neural tube : the teg- 

 ini'titum and crusta become 

 formed as thickenings along 

 the same lamina : the vesi- 

 cle itself becomes the aque- 

 duct of Sylvius. 



In the constriction between the third and fourth vesicles (isthmus of His) the 

 fourth nerve takes origin in the ventral plate of the neural tube. 



The second cerebral vesicle (thalameucephalon). It is from this part 

 of the neural tube that the primary optic vesicles are developed in the earliest 

 period, and they are for some time in free communication with its cavity along the 

 hollow optic stalks. But with the formation of the optic nerves and optic tracts, 

 the stalks become solid, and are, moreover, connected posteriorly with the mid-brain 

 by a prolongation backwards of the tracts. The optic thalamus of each side is 

 formed by a thickening of the lateral wall of the vesicle (figs. 74, 78). The interval 

 between the thalami forms the cavity of the third ventricle. Across it the grey 

 commissure subsequently stretches. The floor becomes prolonged downwards into 

 the infundibulum, and takes part in the formation of the pituitary ~body (figs. 69, 74). 

 The roof, on the other hand, becomes like that of the fifth vesicle, thin and expanded, 

 and remains as a single layer of flat epithelium cells inflected into the ventricle and 

 subsequently occupied by vascular growths of pia mater (choroid plexus of third 

 ventricle, fig. 69, ch 3 ). But at the posterior part of the roof there is a transverse 

 thickening to form the posterior commissure, and in front of this the roof grows 

 upwards and forwards, but subsequently backwards (in man) as a hollow median 

 process to form the pineal gland (epiphysis cerebri). The median process soon takes 

 on a tubular shape (fig. 69, pin), and, after a time, becomes branched, and forms a 

 number of tubular follicles lined by ciliated epithelium, and invested by vascular pia 

 mater. These follicles tend, in man and mammals, as development proceeds, to 

 become solid and occupied by calcareous deposit. But in some reptiles the pineal 



F 2 



