772 THE CENTRAL AXIS OF TEE NERVOUS SYSTEM. 



outwardly, to that portion of the optic layer named the corpus gmiculatum 

 internum ; while they limit, externally, the bands of Reil. 



The anterior corpora quadrigemina, or tuhercula nates, are distinguished from 

 the preceding not only by their larger volume, but by their colour, which is grey, 

 that of the testes being white. They are also rounder, nearer each other, and 

 covered by the cerebral hemispheres ; while the posterior rather lie beneath the 

 cerebellum. A curved groove isolates them, in front, from the thalami optici. 



7. Thalami Optici (Fig. 425, 9). 



This name is given to that part of the upper face of the isthmus which is 

 situated in front of the corpora quadrigemina. These thalami are, therefore, 

 placed above the anterior part of the cerebral peduncles. 



Larger altogether than the corpora quadrigemina — and more so before than 

 behind— each exhibits a grey, slightly convex, and very irregularly quadrilateral 

 surface, covered by the velum interpositum, which separates it from the cornu 

 Ammonis {pes hippocampi), and from the posterior pillars of the fornix, beneath 

 which they are placed. 



Imvardly, they incUne towards each other in forming on the middle line 

 a somewhat deep fissure, in which runs, from before to behind, two white 

 longitudinal bands that will be noticed hereafter as the anterior peduncles of the 

 pineal gland. This fissure enters, behind, the common posterior opening {foramen 

 commune posteriiis) ; in front, into the common anterior opening {foramen commune 

 anterius) — orifices which will be described with the interior of the isthmus. 



Outwardly, each thalamus opticus shows two prominences called the corpora 

 genicidata, from which arise the second pair of nerves ; placed one before the 

 other, the posterior nearer the middle line than the anterior, these two projections 

 are distinguished as external and internal. The corpus geniculatum externum is 

 always more voluminous, better defined, and situated on a more elevated plane 

 than the corpus geniculatum internum, Avhich is united to the posterior corpora 

 quadrigemina by an oblique band (Figs. 425 ; 426, 10, 11). 



Behind, the thalami optici appear to be notched to receive the nates, which 

 they slightly enclose. 



In front, they are separated from the coi'pus striatum by a groove, at the 

 bottom of which is a narrow strip named the semicircular hand {tcenia semi- 

 circular is). 



8. Pineal Gland, or Conarium (Epiphysis Cerebri) ^ (Fig. 425, 14). 



This name has been given to a small tubercle of a reddish-brown colour, in 

 the form of a pine-cone, enveloped by a duplicature of pia mater from the velum 

 interpositum, with its apex upwards, and its base resting on the common 

 posterior opening {foramen of Monro), which it closes, and around which it is 

 attached by a circular ridge. 



From this lamella is detached, in front, two fibrous cords — the anterior 

 peduncles of the conarium (or habenm). These (Fig. 425, 15) are two narrow 



' This organ, and the next to be describerl, do not really belong to the isthmus system, but 

 rather — as has been already mentioned — to the appended glands which should receive separate 

 description. The first is probably the vestige of a rudimentary eye, tlie second that of a 

 lymphatic gland (It appears to be now conclusively established that the pineal bo<ly is the 

 representative of an ancestral form of eye — most clearly demonstrated in certain lizards.) 



