ORGANS OF NEURAL EXCRETION 99 



and physical output, so to speak, may reach its highest 

 level of production, as well as to afford a means of 

 mechanical support, protection, and inhibition to the 

 whole component parts of the systemic nervous system. 



FIG. 29. RIGHT HALF OF THE ENCEPHALIC PEDUNCLE AND CEREBELLUM 



AS SEEN FROM THK INSIDE OF A MEDIAN SECTION. (Allen Thomson 



after Reichert. ) 



II, right optic nerve ; behind it the optic commissure divided ; III, right third nerve; 

 VI, sixth nerve; V3, third ventricle; Th, back part of the thalamus opticus ; 

 ff, section of the pituitary body ; /, pineal gland ; below its stalk is the posterior 

 commissure ; c a, anterior commissure divided, and behind it the divided anterior 

 pillar of the fornix ; / c, lamina cinerea ; z, infundibulum (cavity) ; t c, tuber 

 cinereum ; behind it the corpus albicans ; f, mark of the anterior pillar of the 

 fornix descending in the wall of the third ventricle : c m, commissura mollis ; 

 s p, stria pinealis or peduncle of pineal gland; Q, lamina quadrigemina ; as, 

 aqueduct of Sylvius near the fourth ventricle ; cr, crus cerebri ; P V, pons 

 Varolii ; M, medulla oblongata ; and behind these the cerebellum. 



In what may be called the ''vesicular" stage of develop- 

 ment of the central nervous system, we find that its 

 more solid nerve elements or neurons group or arrange 

 themselves in tubular fashion around a central cavity, 

 which is afterwards to be known as the cerebro-spinal 

 ventricular spaces and central canal respectively, in the 

 lumina of which is secreted by the choroid plexuses the 



