5o8 STRUCTURE OF VERTEBRATES 



grey matter is generally more or less H-shaped in section ; the 

 upper parts (the dorsal columns or horns) consist of association 

 neurones, with their cell bodies, which receive and relay sensory 

 impulses, while the lower parts (the ventral columns or horns) 

 consist of the cell bodies of efferent nerves. The white matter is 

 made up of nerve fibres of various sorts carrying impulses up 

 and down the column. 



The brain is formed as an expansion of the anterior end of 

 the spinal cord. At an early stage this begins to swell, and very 

 soon two flexures or constrictions mark off fore, mid, and hind- 

 brain, or prosencephalon, mesencephalon, and rhombencephalon. 

 These, and the structures directly developed in them, constitute 

 what is called the brain stem. Soon afterwards a lateral outgrowth, 

 which will become olfactory lobe and cerebral hemisphere, 

 appears on each side of the prosencephalon ; the two grow forward 

 and may meet to form a single structure. Together with the 

 original anterior wall of the prosencephalon, the lamina termi- 

 naHs, they make the end-brain, or telencephalon, leaving the 

 remainder of the fore-brain as the tween-brain or diencephalon 

 or thalamencephalon. A dorsal outgrowth of the hind-brain, 

 the cerebellum, makes it possible to distinguish an anterior 

 metencephalon from a posterior after-brain or myelencephalon. 

 There are thus five main subdivisions. Their cavities are called 

 ventricles ; in the hemispheres are two lateral (or first and second) 

 ventricles, each of which opens by a foramen of Monro into the 

 cavity of the diencephalon, or third ventricle. This leads by a 

 channel with the absurd names of aqueduct of Sylvius or iter a 

 tertio ad quartum ventriculum to the fourth ventricle, which 

 extends throughout the rhombencephalon and is continuous with 

 the canal of the spinal cord. The iter is a narrow passage in 

 mammals, but is much larger in lower vertebrates. 



The chief parts of the brain are shown diagrammatically in 

 Fig. 469. The roof of the tween-brain bears a number of out- 

 growths ; the paraphysis is seldom present in adults, and it is 

 rare for the pineal and parietal (or parapineal) bodies, both of 

 which were originally eyes, to be well developed together. The roof 

 between these outgrowths, like that of the myelencephalon, is 

 thin and vascular, making a choroid plexus. From the floor of 

 the tween-brain the infundibulum grows down to become the 

 pars nervosa of the pituitary. Most of the other parts of the brain 

 are thickenings in the walls. It will be seen that this diagrammatic 



