110 THE VOYAGE OF H.M.S. CHALLENGER. 



peduncle. Sir Richard Owen makes use 1 of this and other facts connected with the 

 extension of the pineal body into or towards the cranium in these Fishes and in 

 Reptdes in support of the hypothesis that the conario-hypophysial tract represents the 

 passage of the gullet to the neural aspect of the body and the formation of a neural 

 mouth. But additional interest has quite recently attached to the pineal body by the 

 discovery, as the result of independent research, both by H. W. de Graaf 2 and W. Baldwin 

 Spencer 3 during the year 1 886 of a mesial pineal eye in the Lacertilia. By these naturalists 

 the mesial foramen in the parietal bone in this group of Reptiles has been seen to be occu- 

 pied by an eye, and Mr. Spencer has worked out in a number of species of Lizards the 

 structure of this eye and its connections, from which it would appear that the pineal eye 

 is connected by an elongated stalk or peduncle with the thalamencephalon. This peduncle 

 grows out of the optic thalami ; at first it passes upwards in the interval between the 

 cerebral hemispheres and the optic lobes, and then runs forwards on the dorsal aspect of 

 the cerebrum, to end in the mesial eye, situated in the parietal foramen. 



In the Mammalia this apparatus has practically disappeared, and is represented only 

 by the aborted structure which we call the pineal body, though it should be stated that 

 in the Horse, as M. Chauveau has pointed out, 4 it may occasionally assume larger dimen- 

 sions, and project backwards so as almost to touch the cerebellum. But in the Seals to 

 some extent, and in the Walrus in a more remarkable manner, the pineal body has retained 

 a greater magnitude than is customary in Mammals. The direction, however, which this 

 body takes in these Mammalia is different from that of the stalk of the pineal eye in the 

 Lizards. For in these Reptiles the direction of the peduncle is at first upwards and then 

 forwards, so as not to overlie either the optic lobes or the cerebellum, whereas in the 

 Walrus and Seals the direction of growth is always backwards. Two factors may operate 

 in the cranial cavity of the Walrus and Seals to induce the backward direction to which I 

 have referred, viz., the growth of the tense unyielding tentorium, and the backward 

 development of the hemispheres of the cerebrum. Through lying subjacent to the ten- 

 torium the growth of the elongated pineal body in the direction either of the parietal 

 bone or of the superior part of the occipital bone is effectually prevented, and the only 

 course which it can take is towards the cerebellar region of the occiput. Similarly the 

 posterior development of the cerebral hemispheres, which overlie both the optic lobes and 

 the cerebellum, would by the compression of the pineal body between the cerebrum and 

 cerebellum assist in giving it a backward direction. It is possible, also, that these same 

 factors may operate in producing the aborted condition of this body which one finds in 

 the Mammalia as compared with the Lizards. For the pressure exercised by the growth 



1 Rep. British Assoc, York, 1881, p. 719 ; Aspects of the body in Vertebrates and Invertebrates, Lond., 1883. 



2 Zool. Anzeiger, March 29, 1886. 



3 Quart. Journ. Micr. Sri., October 1886. 



4 Traite dAnatomie compared des animaux domestiques, 185", p. 650, fig. 177. 



