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 epicoele, 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 (metacoele, fig. 156). 



FIG. 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; i, infundibulum; M, mid-brain; N, nuchal flexure; P, pontal flexure; RO, optic 

 recess; T, '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- 



