DEVELOPMENT OF THE PINEAL SYSTEM OF BIRDS 291 



No nerve-fibres connecting the pineal organ with the brain are present 

 either within or near the stalk ; the nerve-fibres of the habenular and the 

 posterior commissures are, therefore, derived from sources entirely 

 outside the pineal system, and in those cases in which the pedicle has 

 been ruptured it is obvious that all nervous connections with the brain 

 by means of it, should they have existed in embryonic life, will have 

 disappeared in the adult. 



The pineal organ of birds, however, does not always conform to the 

 type described above ; it is often a simple elongated tube, arising in the 

 usual situation between the habenular and posterior commissures and 

 extending upwards towards the vault of the skull, beneath which it ends 

 in a slightly lobulated expansion (Fig. 199). This is, however, not regarded 



Hem 



Fig. 199. — Parietal Region and Cerebellum of a Sparrow. (After Gaze.) 

 Cbl. : cerebellum. Ep. : epiphysis. 



Cp. : posterior commissure. Hem. : hemisphere. 



as an end vesicle. The tube is hollow and communicates in young 

 specimens with the cavity of the third ventricle. It occupies the narrow 

 interval between the cerebellum, which lies behind, and the two hemi- 

 spheres, which are in front and lateral. Owing to the large size of the 

 hemispheres and cerebellum in birds, the tube is of considerable length. 

 It is invested by a fibrovascular sheath continuous with the pia mater 

 at its attachment to the brain. 



The Development of the Pineal System of Birds 



This has been studied chiefly in the chick, and since the early stages 

 of development in the chick do not differ essentially from those of other 

 types of bird, we shall confine our description to that of the chick, and 

 only refer to special points in the development of the system in other 

 birds when these have some bearing on the general problem we are 

 considering. 



