IX BRAIN 331 



portion grows out into irregular projections surrounded by blood-sinuses 

 and forming the saccus vaseulosus. 



Attached to the ventral surface of the infundibulum is the pituitary 

 body (Fig. 138, B, pit), elongated in shape in an antero-posterior direction. 

 This, although in the adult condition it appears to be a part of the brain, 

 is in reality of quite independent origin. It originates as a pocket-like 

 ingrowth of the ectoderm on the ventral side of the head. This grows 

 inwards underneath the brain and presently becomes isolated from the 

 outer skin, forming a closed sac immediately underlying and apparently 

 forming part of the infundibulum. Whereas the pituitary body would 

 appear to have been originally a gland opening either into the buccal 

 cavity or close in front of it, its function is in the typical modern verte- 

 brate that of a ductless gland. Its contribution to the internal medium 

 is clearly of importance since in the higher vertebrates disease of the 

 pituitary is accompanied by disturbances of the general metabolism 

 which find their expression in peculiarities of growth (acromegaly) and 

 eventually in serious disease, and in Tadpoles it has been found that the 

 absence of its internal secretion paralyses the activity of the chromato- 

 phores of the skin. 



The Hemispheres are in the typical vertebrate a pair of bulging 

 outgrowths from the wall of the thalamencephalon close to its front end. 

 Each of them contains a cavity, the lateral ventricle, which is simply a 

 prolongation of the third ventricle and remains throughout life con- 

 nected with it by a small opening, the Foramen of Monro, named after 

 Monro (" secundus ") of Edinburgh, one of the great pioneers of verte- 

 brate anatomy. The wall of the hemisphere contains ganglion-cells 

 especially associated with the sense of smell and it is regarded as a 

 portion of the brain which has been evolved specially in connexion with 

 that sense. 



The most marked peculiarity of the Hemisphere region of the Dogfish 

 (Fig. 138, A, h) is that in this animal the paired condition is obscured, 

 the two hemispheres being continuous across the mesial plane, instead 

 of being separated by a cleft as is the case with most vertebrates. 



The portion of hemisphere wall lying next to the olfactory organ 

 or organ of smell comes to bulge out into a somewhat T-shaped 

 olfactory lobe. The cross-piece of the T contains the ganglion-cells 

 immediately connected with the organ of smell and is known as the 

 olfactory bulb (Fig. 138, o.T) while the narrower stalk (olfactory or bulbo- 

 olfactory tract) consists mainly of nerve-fibres passing back to a second 

 set of ganglion-cells within the body of the hemisphere (olfactory 

 tubercle). 



