174 ESSENTIALS OF ZOOLOGY 



sympathetic ganglia, and the cortical forming a long structure 

 on each side, just internal to the kidney. 



The central nervous system arises from the medullary canal 

 of the embryo. The brain is defined by three vesicles, the 

 fore-, mid-, and hind-brain, which originate as expansions of the 

 anterior end of the neural canal. The forebrain is deflected 

 below the midbrain, the roof of which comes thus to occupy 

 the anterior end of the embryo. During this process, which is 

 called the mesencephalic flexure, the forebrain gives off paired 

 diverticula to form the cerebral hemispheres and the optic 

 cups. The cerebral hemispheres, or prosencephalon, are at 

 first hollow, and the cavities they contain are the lateral ven- 

 tricles ; but the walls thicken by the development of neurons 

 which form areas of grey matter and of nerve fibres which 

 form tracts of white matter. The hemispheres arise primitively 

 in association with the nasal cavities, and anteriorly they give 

 off the olfactory lobes which extend forwards and outwards 

 to end over the nasal sac on each side. As the result of the 

 thickening, the cavities or ventricles are obliterated, and only 

 an indication of their communication with the original cavity 

 of the forebrain by the foramina of Monro can be made out. 



The forebrain, after giving off the optic cups and the 

 hemispheres, is termed the thalamencephalon, and its cavity the 

 third ventricle. The side walls are thickened, forming the 

 optic thalami. The front wall is the lamina terminalis, the 

 original anterior end of the neural canal. The roof is thin and 

 is produced into the median pineal body or epiphysis, and this 

 remains in the adult as a fine process of the roof. The floor 

 is also thin and is folded into the lobi inferiores and produced 

 into the infundibulum, to which is attached the pituitary body 

 or hypophysis. Between these structures of the floor the 

 three-lobed sacculosus vasculosus may be seen. Transverse 

 fibres connect the thalami antero-ventrally by the anterior 

 commissure, and behind the pineal outgrowth of the roof by 

 the posterior commissure. 



The midbrain, or mesencephalon, is expanded dorsally and 

 folded longitudinally to form the paired optic lobes. Below, 

 it is thickened like the remaining part of the brain, and the 

 thick diverging columns of the floor of the midbrain are called 



