18/(3.] PROF. T. H. HUXLEY ON CERATODTJS EORSTERI. 29 



-The brain of Ceratodus nearly fills the cranial cavity, the inter- 

 space left between it and the walls of the latter being, to a great ex- 

 tent, occupied by a peculiar reticulated tissue. The medulla ob- 

 longata is long and slender, but widens, anteriorly, in the region of 

 the fourth ventricle. This cavity is arched over by a tela vasculosa 

 (To, figs. 1 and 2), separated into two lateral convexities by a slight 

 median depression. In front, each convexity is continued into a blind 

 rounded cornu, which lies over the origin of the fifth and seventh 

 auditory nerves. The two cornua diverge, and the cerebellum is 

 continued backwards as a triangular lamella between them. The 

 cerebellum is relatively very small, being represented merely by the 

 thin arched roof of the anterior part of the fourth ventricle. In 

 front of it is a rounded elevation, obscurely divided by a longitudinal 

 depression into two. These are the only indications of the optic 

 lobes of the mid brain, or mesencephalon. In front of this is the 

 fore brain. The hinder division (or thalamencephalon) is narrower 

 than the mesencephalon, and passes below into the infundibulum, 

 which terminates in the large, oval, flattened, pituitary body (hypo- 

 physis). This is lodged in an excavation of the cartilaginous floor 

 of the skull representing the sella turcica. 



Dorsally, the thalamencephalon is continued upwards and forwards 

 into the subcylindrical peduncle of the pineal gland (epiphysis or 

 conarium) . This is a large heart-shaped body, the base of which 

 is turned downwards and backwards. The apex is connected by 

 fibrous and vascular tissue with a depression in the cartilaginous roof 

 of the skull. 



Next follows the largest division of the brain, answering to the 

 cerebral hemispheres and the olfactory lobes. The former are repre- 

 sented by a single oval lobus communis, the middle area of the roof 

 of which is occupied by a broad thick tela vasculosa. From the 

 anterior dorsal aspect of the prosencephalon proceeds, on each side, 

 the large olfactory lobe, which, flattening in front, and becoming 

 obliquely truncated, terminates against the posterior walls of the 

 olfactory sacs. A backward prolongation of the mesethmoid carti- 

 lage separates the two lobes. In the lateral view of the brain (fig. 2) 

 the manner in which the olfactory lobes take their origin from the 

 dorsal aspect of the prosencephalon is well seen. The short and 

 thick infundibulum, terminating in the flattened oval pituitary body, 

 and the origins of the second, third, fifth, eighth, ninth, and tenth 

 nerves are shown. 



The ventral view (fig. 3) displays the origins of the small optic 

 nerves (II) which arise close together from the floor of the thala- 

 mencephalon. Whether any chiasma exists could not be ascertained. 



In the middle line of its ventral aspect the prosencephalon presents 

 a deep longitudinal fissure, lodging anterior cerebral arteries. The 

 walls of the fissure have been separated by turning the left division 

 of the prosencephalon to one side; and the floor of the ventricle (v.c), 

 which is contained in the prosencephalon, has been removed. 



"When the dorsal wall of the brain was cautiously laid open by 

 a median section, it was found to contain one large ventricular cavity 



