232 



THE PINEAL ORGAN 



or even the whole stalk are frequently found to be absent in adult speci- 

 mens, a circumstance which suggests that the development of nerve- 

 fibres when it occurs is merely a transitory phase in the existence of a 

 degenerate organ which is variable in the degree of differentiation to 

 which it attains in different examples of the order or even in individuals 

 of the same species. The occasional presence of nerve-fibres in the stalk 

 of the end vesicle, although not constant, is of morphological importance, 

 and the following detailed description by Braem (1898) of both the extra- 

 cranial and intracranial course of the pineal nerve is especially interesting. 

 He distinguishes between the " nervus pinealis " of the parietal organ and 

 the " tractus pinealis " of the epiphysis (Fig. 166). 



Dealing first with the pineal nerve of Rana, Braem states that it consists 

 of thick medullated fibres accompanied by blood-vessels. These fibres 

 enter the organ from below, either at the middle or near the hinder pole. 

 Having entered the vesicle they gradually become imperceptible and are 

 lost. Connective tissue elements enter slightly into the composition 

 of the nerve, and Studnicka mentions that he has found pigment cells on 

 the surface of the nerve and, further, that he considers the elongated 

 nuclei seen in the course of the nerve to be connective tissue in nature. 

 In any case, there is at least a fine endothelial sheath, which separates 

 Ep. Tr.pin. C.p. MB. 



Fig. 166. 

 A — Sagittal section through the Epiphysis (proximal part of the pineal organ) 



showing the pineal tract, from an adult example of Rana temporana. 

 B — Transverse section through the epiphysis and roof of the diencephalon. 



Ch. H. : Chiasma habenularis. MB. : midbrain. 

 C.p. : commissura posterior. Tr. pin. : tractus pinealis. 



Dien. : diencephalon. ///. V. : third ventricle. 

 Ep. : epiphysis. 



(After Braem, 1898.) 



the^ nerve and the accompanying blood-vessels from the cranio-dorsal 



lymph space in which the greater part of the nerve lies. Braem further 



describes the nerve as passing through the lymph sac and sooner or later 



piercing the skull in an oblique direction between the two frontal (fronto- 



