THE BRAIN. 



437 



know almost nothing, beyond that a pathological state of this organ 

 is associated in man with certain diseases, e.g., acromegaly. 



The Pineal Body. — The dorsal upgrowth (or epiphysis) from the roof 

 of the thalamencephalon is represented, though to a varying extent, in 

 all Vertebrates. It is terminally differentiated into a little body known 

 as the pineal body. This was entirely an enigma until De Graaf dis- 

 covered its eye-like structure in Angitis, and Baldwin Spencer securely 

 confirmed this in the New Zealand "lizard" (Sphenodon) where the 

 pineal body shows distinct traces of a retina. 



In Elasmobranchs the pineal process is very long, and, perforating 

 the skull, terminates below the skin in a closed vesicle. In the young 

 frog it also comes to the surface above the skull, but degenerates in 

 adolescence. In Sphenodon the stalk passes through the skull by the 

 " parietal foramen," so that the " eye " itself lies close beneath the skin, 

 the scales of which in this region are specialised and transparent. 



In Iguana, Aitgiiis, Lacerta, &c. , the 



1 — 7 — ■ — 7r^, epiphysis loses connection with the 



"eye" portion; and it is also to be 

 noticed that in Angitis and Iguana the 

 pineal body receives a nerve from a 

 "parietal centre" near the base of, 

 but independent of, the epiphysis ; this 

 nerve is transitory in Angnis, more or 

 less persistent in Iguana. Above Rep- 

 tiles the pineal stalk is always relatively 

 short, and its terminal portion forms a 

 glandular structure. In fact, the de- 

 velopment of the pineal body is much 

 more complicated than at first appeared ; 

 thus according to Locy's researches on 

 Acanthias embryos, it represents the 

 fusion of an extra pair of eyes. 



The full significance of the pineal 

 body is thus uncertain. According to 

 one view, its primitive function is that 

 of an unpaired, median, upward-looking 

 eye — a function retained only in the 

 Reptiles mentioned above, the organ 

 having elsewhere undergone (inde- 

 pendent) degeneration. It has also been interpreted as an "organ for 

 the perception of warmth." It may be, however, that the function dis- 

 cernible in some Reptiles is not primitive, but the result of a secondary 

 modification of the structure in question. Thus, one of first inter- 

 pretations (Dohrn's) connected the pineal and the pituitary outgrowths 

 with a supposed passage of the original hypothetical mouth through the 

 nerve cord. 



The second primary vesicle of the brain undergoes little 

 alteration, and forms the third region, that of the optic lobes 

 (mesencephalon or mid brain) in the adult brain. The floor 



Fig. 140. — Diagram of the 

 parts of the brain in Verte- 

 brates. (After Gaskell. ) 



c.h.. Cerebral hemispheres ; 

 c.pL, choroid plexus; (J. M., optic 

 thalami ; oL, optic lobes ; cb., 

 cerebellum ; c.pl., choroid plexus ; 

 M.O., medulla oblongata ; s.c, 

 spinal cord. 



