858 THE XERVE SYSTEM 



sagittally placed, slit-like form of the so-called third ventricle. The cavities oi 

 the optic and olfactory buds become obliterated. 



II. Mid-brain. The second primary vesicle becomes somewhat later differen- 

 tiated and takes a less prominent part in the adult brain. Its dorsal wall goes 

 into the formation of four eminences, the corpora quadrigemina, while the lateral 

 and ventral sections grow considerably to form the crura cerebri. The neura 

 cavity within the mid-brain persists as the narrow aqueduct joining the third anc 

 fourth ventricles. 



III. Hind-brain. The third primary brain vesicle is demarcated from the mid- 

 brain by a marked constriction to which has been given the term isthmus rhomb 

 encephali. The hind-brain is specially characterized by the great expansion o 

 its thinned-out, membranous dorsal wall caudad, while cephalad the dorsal wal 

 becomes very much thickened as the proton or fundament of the cerebellum. 



The ventral and lateral parts undergo thickening to form the pons and medulh 

 oblongata. 



Forebrain. 



Imagination of Ectoderm 

 to form the lens rudiment' 



Neural part 

 of retina. 

 Optic stalk. 



FIG. 626. Trans-section of head of chick embryo of forty-eight hours' incubation. X 55. 

 (From Duval's Atlas d'Embryologie.) 



Flexures of the Brain Tube. The difference in growth rate of the differ- 

 ent parts of the brain tube and the marked disproportion between the rapic 

 brain growth and slower head growth causes the encephalic neural tube to becom< 

 sharply bent upon itself at certain points. The first flexure to occur is involvec 

 in a bending of the entire head and takes place in the region of the mid-brain 

 this flexure is termed the cephalic flexure. A second bending of the tube occur; 

 at the junction of the spinal cord and hind-brain; this is termed the cervical flexure 

 and is so pronounced in the fifth week of intrauterine life that the brain tube anc 

 spinal cord form a right angle with each other. A third flexure is produced, ir 

 consequence of the other two, in the region of the future pons, and is therefore 

 called the pontile flexure. Subsequently the cervical and pontile flexures an 

 obliterated by a gradual straightening of this portion of the brain axis. 



Dorsal and Ventral Laminae or Longitudinal Zones of the Brain. Quite 

 like the longitudinal division of the developing spinal cord, there is a differ- 

 entiation of the brain tube into dorsal and ventral zones, though much less clearlj 

 shown. The limiting furrow between the two is not demonstrable in the fore- 

 brain; at least it is disputed, on good grounds, that it exists there. It is claimec 

 even that three such longitudinal divisions exist on each side (Kupffer) and the 

 formation of the cranial nerves is not quite comparable to the spinal nerves 

 although there is a fair homology with their dorsal-sensor and ventral-motor func- 

 tional differentiation, despite their frequent admixture in some cranial nerves 01 

 the total absence of the one category in others. Thus the motor elements dc 

 not extend higher than the mid-brain and the dorsal division preponderates 

 in the more highly organized parts of the brain, becoming predominant in the 

 higher vertebrate species particularly in man. 



