408 THE PINEAL ORGAN 



which is capable of being influenced by impulses reaching it through 

 its afferent nerve-fibres and transmitting such impulses by efferent fibres 

 (e.g. sympathetic) to the organs or regions to which these nerves are 

 distributed. 



A general survey of the comparative anatomy of the pineal region, 

 with detailed descriptions of the nerve cells and nerve-fibres of the pineal 

 system in special types of animals, was published in 1905 by Studnicka 

 Die Parietalorgane. Oppel. Teil V.), and recent accounts with references 

 to the literature in such works as UEpiphyse, by J. Calvet, a special article 

 on the pineal gland by del Rio Hortega in Cowdry's Special Cytology, 

 Penfield (1928), and various articles such as those by Beraneck, Clarke, 

 Darkschewitsch, Dendy, Dimitrowa, Herring, Pastori, and others. It will 

 be realized on studying these contributions to the innervation of the 

 pineal system that substantial agreement has been reached on the following 

 points : 



1. Tracts of nerve-fibres described as the nervus pinealis, nervus 

 parietalis, tractus pinealis, and tractus habenularis have been traced from 

 receptive sensory cells or ganglion cells in the retina of the parietal organs 

 (namely, the pineal eye and end-vesicle of the parapineal organ) and 

 found to terminate in or traverse the habenular ganglia, the superior and 

 posterior commissures, and Meynert's bundles. These fibres have been 

 observed in cyclostomes and other fishes, amphibia, and reptiles. They 

 may be situated in the stalk of the vesicle, and thus resemble the optic 

 nerve-fibres of the lateral eyes of vertebrates ; or they may course as an 

 independent tract through the areolar connective tissue in the neighbour- 

 hood of the stalk ; or, after the disappearance of the stalk, they may lie 

 in the region formerly occupied by the stalk. The nerve-fibres may be 

 present only in the larval stages or they may persist in the adult animal. 



2. Similar tracts of nerve-fibres may arise from sensory cells in the 

 wall of the pineal sac or the epiphysis in elasmobranch and teleostean 

 fishes, amphibia, reptiles (saurians and snakes), and in mammals. These 

 fibres terminate for the most part in the posterior commissure, but con- 

 nections are established in some species also with the internal capsule, 

 stria; medullares thalami, Meynert's bundles, habenular commissure 

 and ganglia, and the optic tracts (Darkschewitsch). 



There is, however, a considerable amount of variation in different 

 species of mammals, e.g. Herring states that occasional nerve-fibres 

 may enter the pineal body from the habenular commissure in the cat, 

 monkeys, and man, but have probable no functional significance ; whereas 

 in the rat the pineal body is anatomically widely separated from the 

 habenular commissure, and no nervous connection persists between 

 them. In the adult rat the pineal body is an isolated organ which lies 



