THE INTER-BRAIN. 671 



forms another and smaller recess, which extends into the stalk of the pineal gland, 

 and is termed the pineal recess. At its upper and anterior part, immediately be- 

 hind the anterior pillars of the fornix and in front of the optic thalamus, is an 

 opening, the foramen of Monro, by which this ventricle communicates with the 

 lateral ventricle on either side. The roof of the cavity is limited in front and 

 behind by transverse bands of white matter, known respectively as the anterior 

 and ]>nsterior commissures. The former has already been described in connection 

 with the corpus striatum (page 665). 



The middle or soft commissure consists almost entirely of gray matter. It 

 connects the two optic thalami, and is continuous with the gray matter lining the 

 anterior part of the third ventricle. It is frequently broken in examining the 

 brain, and might then be supposed to be wanting ; it is sometimes double. 



The posterior commissure is a rounded band of white fibres, which stretches 

 across from one optic thalamus to the other, overlying the upper end of the 

 aqueduct of Sylvius, or iter a tertio ad quartum ventriculum. It is usually 

 described as belonging to the inter-brain, but would appear to belong in part to 

 the mid-brain, since some of its fibres are commissural and connect the anterior 

 corpora quadrigemina to the fillet of the opposite side (see below). In addition 

 there are other decussating fibres, which come from the tegmentum of the crus 

 cerebri on one side and decussate with those of the opposite side in the posterior 

 commissure, and passing through the optic thalamus reach the cerebral hemispheres. 

 Fibres have also been described as taking their origin in the pineal body and 

 ganglion habenuhe, and passing across to the posterior longitudinal bundle and 

 oculomotor nucleus of the opposite side ; these fibres occupy the ventral part of 

 the commissure, and receive their myelin sheath before those in its dorsal part. 

 But to a certain extent the posterior commissure belongs to the inter-brain, since 

 it contains fibres which serve as commissural fibres between the two optic thalami. 



The optic thalami are two large oblong masses, situated on either side of the 

 third ventricle, and lying between the diverging portions of the corpora striata. 

 They are composed mainly of gray matter, but their free surfaces are coated with 

 a thin layer of white nervous tissue. They present outer and under surfaces, 

 which are not free, but are blended with contiguous parts of the brain, and upper, 

 inner, and posterior surfaces, which are free. The anterior extremity is narrow, 

 and forms the posterior boundary of the foramen of Monro. The outer surface 

 is in contact Avith the posterior limb of the internal capsule, Avhich separates it 

 from the lenticular nucleus. The inferior surface rests upon and is continuous 

 with the tegmentum of the crus cerebn. Its upper surface is free, and is separated 

 from the caudate nucleus by a furrow Avhich lodges the lamina cornea, the vein of 

 the corpus striatum, and the taenia semicircularis. It is divided into an outer and 

 an inner part by a groove which runs from behind, forward and inward* The 

 outer part forms a portion of the floor of the lateral ventricle, and is covered by 

 the ependyma of that cavity ; it terminates in front in a tubercle, the anterior 

 tubercle of the optic thalamus. The inner part is covered by the velum inter- 

 positum, which separates it from the fornix, and is excluded from both the lateral 

 and third ventricles by the reflection of the lining of these cavities, and is there- 

 fore destitute of an ependymal covering. 



The internal surface forms the lateral Avail of the third ventricle, and running 

 along its upper border is the peduncle of the pineal gland, from Avhich the 

 ependyma of the third ventricle is reflected on to the under surface of the velum 

 interpositum. The posterior surface projects beyond the level of the corpora 

 quadrigemina, and forms a well-marked rounded prominence, the posterior tubercle 

 or pnlrinar. The pulvinar is continued externally into a second eminence, the 

 external geniculate body, which is placed above and to the outer side of the internal 

 !/>'>/ iculate body, and from which it is separated by the superior brachium, one of 

 the roots of the optic tract. 



The optic thalamus is formed chiefly of gray matter, which is arranged in tAvo 

 masses, the outer and inner nuclei, and these are partially separated from each 



