THE PINEAL GLAND 305 



find in the former group at least some indication of a pineal 

 sense-organ, even in the adult, but such is not the case. On the 

 other hand, we find in the snakes a condition of the epiphysis 

 somewhat resembling that which obtains in birds and mammals ; 

 the organ being represented by a highly vascular solid body, 

 which may perhaps be regarded as a ductless gland. In the 

 Crocodilia the degeneration of these organs appears to have been 

 carried to its extreme length, and neither in the embryo nor in 

 the adult can we recognise any trace of an epiphysis whatever. 



In birds, again, all traces of the pineal sense-organs appear 

 to have disappeared. The epiphysis in this group may be 

 either tubular, follicular, or solid, and is usually connected with 

 the brain by a hollow or solid stalk. It consists (in Meleagris) 

 of a vast number of small follicles lined by ependymal 

 epithelium and separated from one another by vascular con- 

 nective tissue, and Studnicka has demonstrated the presence 

 of ependymal cells with extremities projecting into the cavities 

 of the follicles. Here again we are probably dealing with a 

 ductless gland which passes its secretions into the blood. The 

 epiphysis in this group is generally stated to arise in the embryo 

 as a single outgrowth of the roof of the thalamencephalon, but 

 indications of a paired origin have been observed by several 

 investigators, especially Cameron. 



Lastly, amongst the Mammalia we find a condition very 

 similar to that which obtains in birds— the epiphysis being 

 represented by a presumably glandular organ, made up of a 

 vast number of follicles separated from one another by vascular 

 connective tissue, and lined by ependymal epithelium. Here, 

 once more, we probably have before us a ductless gland of 

 unknown function, formed most likely by the metamorphosis 

 of the " stalk," or a portion of the " stalk," of one or both of a 

 primitive pair of epiphysial outgrowths. A remarkable feature 

 of this gland in the human subject is its function of secreting 

 the so-called " brain-sand," consisting of concretions of carbonate 

 and phosphate of lime and magnesia, but whether or not this 

 secretion can be legitimately compared to the formation of the 

 " white pigment " in the pineal eye of the lampreys, as has been 

 suggested, must be regarded as extremely doubtful. 



It would appear, therefore, that at the present day we actually 

 know no more than did Descartes of the function of the pineal 

 gland in the highest vertebrates. We know much, however, of 



