156 



COMPARATIVE ANATOMY 



IT 



roof of the latter (valve of Vieussens) is nervous, and its floor 

 becomes greatly thickened. The greater number of the cerebral 

 nerves arise from the medulla oblongata, so that its physiological 

 importance is very great. The cerebellum 

 may become more or less distinctly sub- 

 divided into lobes. 



In the course of the development of the 

 brain the walls of the cerebral vesicles be- 

 come more and more thickened, so that 

 their cavities undergo a gradual constriction. 

 A series of unpaired ventricles (prosoccele, 

 thalamoccele, mesocaele, metaccele, myeloccele, 

 see p. 153), lying in the longitudinal axis of 

 the brain, as well as paired outgrowths from 

 certain of them, can always be distinguished 

 (Fig. 127). When cerebral hemispheres are 

 developed (as is generally the case), the 

 prosoccele gives rise to paired cavities, ex- 

 tending into them, and known as the lateral 

 ventricles (ventriculus 1 and 2); each of these 

 communicates with the thalamoccele or 

 third ventricle by means of an opening, the 

 foramen of Monro, and may be continued 

 into the corresponding olfactory lobe as a 

 rhinoccelc or olfactory ventricle. Each optic 

 lobe also usually contains an optic ventricle, 

 or optoccelc, communicating with the meso- 

 ccele or aqueduct of Sylvius. There may be a 

 distinct metacoele in the cerebellum opening 

 into the myeloccele or fourth ventricle. 



FIG. 127. DIAGRAM OF 

 THE VENTRICLES OF THE 

 VERTEBRATE BRAIN. 



VH, cerebral hemispheres 

 containing the lateral 

 (1st and 2nd) ventricles 

 (8V) ; ZH, thalamen- 

 cephalon,with the third 

 ventricle (///) ; a thick- 

 ened vascular part of 

 the pia mater (choroid 

 plexus) roofs over the 

 third and fourth ven- 

 tricles ; each lateral 

 ventricle communicates 

 with the third ventricle 

 by a small aperture, the 

 foramen of Monro ( FM) ; 

 MH, mid-brain, which 

 encloses the aqueduct 

 of Sylvius (Aqjf com- 

 municating between the 

 third and fourth vent- 

 ricles ; HH, cerebel- 

 lum ; Nil, medulla ob- 

 longata, enclosing the 

 fourth ventricle (IV) ; 

 Cc, central canal of the 

 spinal chord (7?). 



A so-called fifth ventricle, situated between the 

 corpus callosum and fomix, is found in Mammals, 

 but morphologically it has nothing to do with the 

 ventricles proper, and simply represents a space 

 between the thin internal walls (septa lucida) of 

 the two hemispheres. 



All five cerebral vesicles lie at first in 

 the same horizontal plane, but in the course 

 of development a cerebral flexure takes place, 

 the axis of the vesicles becoming bent down- 

 wards, so that at a certain stage the mesen- 

 cephalon forms the apparent apex of the 

 brain. In Mammals, the parts of the brain 

 become still further folded on one another, so that a parietal, a 

 Varolian, and a cervical bend may be distinguished (Fig. 128) : 

 this process is connected with the further development of the skull 

 and the rapid longitudinal growth of the brain. 



