324 THE PINEAL ORGAN 



with the rounded base of the cone, corresponding to the root of attach- 

 ment, lying just above the posterior commissure and the apex directed 

 backwards. In some cases, however, as in the rabbit, it may consist of a 

 terminal pyriform expansion which is attached by a long, narrow stalk 

 to the roof of the third ventricle in the usual situation (Fig. 225) ; while 

 in the rat the terminal vesicle becomes separated by rupture of the stalk, 

 and in the adult, as pointed out by Herring, all communication with the 

 habenular ganglia is cut off and the only connections of the organ with 

 the body generally are by means of the vascular and sympathetic systems. 



pit 



Fig. 224. — Diagram of a Median Section through the Forepart of a Brain 

 of a fcetal ornithorynchus, showing the plneal body and its relation 

 TO the Habenular and Posterior Commissures. (After G. Elliot Smith.) 

 a.c. : commissura anterior. p.a. : precommissural area. 



b.o. : bulbus olfactorius. par. : paraphysis. 



ch. 3 : choroid plexus of third ventricle, p.c. : posterior commissure. 

 ep. : epiphysis cerebri. pit. : pituitary gland. 



f.M. : Foramen of Monro. r. inf. : recessus infundibuli. 



hip. : rudiment of the hippocampus. r.o. : recessus opticus. 



hyp. : hypophysis cerebri. s.c. : superior commissure. 



/. inf. : lamina infraneuroporica. vel. : velum. 



opt. n. : optic nerve. 



The pineal organ of the rabbit and of the rat thus differ markedly from 

 those of the sheep, ox, or horse, in which the organ has the usual conical 

 or oval form and is connected by well-defined superior and inferior 

 peduncles with the habenular and posterior commissures (Figs. 226 and 



227). 



Complete separation of the pineal organ from the central nervous 

 system in the adult animal, such as occurs in the rat, precludes any possi- 

 bility of any secretory function being under the direct control of the 

 central nervous system ; and if such a secretion exists, it can only be 

 regulated by the blood circulating through its vessels ; thus it may be 

 supposed that the amount of blood passing through the vessels could be 

 influenced through the sympathetic system, and that the secretory 



