BRAIN. 143 
the cavity of the primitive three (continuation of the central canal of 
the spinal cord), but modified in different ways. The cavity in the 
primitive fore-brain is divided with the outgrowth of the hemispheres 
into three chambers known at ventricles, a pair of cerebral ventricles 
in the hemispheres and a third ventricle in the thalamencephalon. 
The paired ventricles are connected with the third by a pair of narrower 
passages, the foramina of Monro (for. interventriculares). In the 
higher vertebrates the cavity of the mid-brain becomes reduced to a 
narrow tube, the aqueduct (or iter), but in the lower classes (fig. 156) 
this expands dorsally into a cavity, the epiccele, in the upper part of the 
optic lobes. The aqueduct terminates behind in the fourth ventricle 
which lies in the hind-brain, extending forward beneath the cerebellum 
and gradually diminishing in the medulla to the central canal of the 
spinal cord. Sometimes there is a prolongation of the fourth ventricle 
into the cerebellum (metaceele, fig. 156). 
Fic. 148.—Median section of brain of pig 15.5 mm. long, showing flexures of the brain 
C, principal flexure; cs, corpus striatum; CP, chorioid plexus of fourth ventricle; h, hypo- 
physis; 7, infundibulum; M, mid-brain; N, nuchal flexure; P, pontal flexure; RO, optic 
recess; 7’, ’twixt-brain. 
So far the brain has been treated as if it were a continuation of the 
spinal cord in a straight line. In reality, by unequal growth in dorsal 
and ventral zones, it becomes flexed in the vertical plane. In the lower 
vertebrates, these flexures never attain great prominence and largely 
disappear in the adult. They are more developed in the higher groups 
and persist throughout life. Most constant is the primary flexure 
in the mid-brain, by which the derivatives of the fore-brain are bent 
downward at a right angle (or more) to the axis of the rest. Second 
to appear is the nuchal flexure in the hinder part of the medulla ob- 
