510 THE NERVOUS SYSTEM. 
to admit a crow-quill, and through these passages the epithelial lining of the three 
ventricles becomes continuous. From the foramen of Monro a distinct groove on 
the lateral wall of the ventricle leads backwards towards the mouth of the Sylvian 
aqueduct. It is termed the sulcus of Monro, and is of importance, inasmuch as it 
represents in the adult brain the furrow which divides the lateral wall of the 
embryonic brain-tube into an alar and a basal lamina. 
The outline of the third ventricle, when viewed from the side in a mesial section 
through the brain, or as it is exhibited in a plaster cast of the ventricular system of the 
brain, is seen to be very irregular. It presents several diverticula or recesses. Thus, in 
the fore-part of the floor there is a deep, funnel-shaped pit or recess, leading down through 
the tuber cinereum into the infundibulum of the pituitary body. Another recess, the 
recessus opticus, leads forwards immediately in front of this, above the optic chiasma. 
Posteriorly two diverticula are present. One, the recessus pinealis, passes backwards 
above the posterior commissure and the mouth of the Sylvian aqueduct for a short 
distance into the stalk of the pineal body. The second is placed above this and is carried 
backwards for a greater distance. It is a diverticulum of the epithelial roof, and, therefore, 
is difficult to demonstrate. It is termed the recessus suprapinealis. 
CEREBRAL CONNEXIONS OF THE Optic TRACT. 
One nerve, the optic nerve or the nerve of sight, is connected with this section 
of the brain. At the optic chiasma the optic nerves of the two sides are joined 
together and a partial decussation of fibres takes place. The fibres which arise in 
the mesial half of each retina cross the mesial plane and join the optic tract of the 
opposite side. The optic tract proceeds backwards round the crus cerebri, and in 
the neighbourhood of the corpora geniculata divides into two roots, viz. a lateral 
and a mesial (p. 491). 
Mesial Root of the Optic Tract—The Commissure of Gudden.—The mesial 
root of the optic tract disappears under cover of the corpus geniculatum internum 
and a large proportion of its fibres arise in this nuclear body. As to the origin of 
the other fibres, we possess at present no precise information. The mesial root, 
although it is composed of fibres which run in the optic tract, has absolutely 
nothing to do with the optic nerve. These fibres, when traced forwards, cross the 
mesial plane in the posterior angle of the optic chiasma and are carried backwards 
in the opposite optic tract, to form on that side its mesial root. The fibres, there- 
fore, are commissural, and constitute a bond of union, called the commissure of 
Gudden, between the internal geniculate bodies. 
Lateral Root of the Optic Tract.—The lateral or true visual root of the 
optic tract is composed of fibres which come—(1) from the lateral half of the retina 
of its own side; and (2) from the mesial half of the retina of the opposite side, and 
which have crossed the mesial plane in the optic chiasma. But in addition to the 
afferent retinal fibres there are a certain number of efferent fibres in the optic 
tract, fibres which take their origin in the brain and end in the retina. These are 
distinguished from the afferent retinal fibres by their exceeding fineness. 
The fibres of the lateral root of the optic tract end in the superior quadrigeminal 
body, in the external geniculate body, and in the pulvinar of the optic thalamus. 
The fibres to the superior quadrigeminal body reach it through the superior brachium 
(p. 493), and for the most part spread out on its surface in the stratum zonale 
before they sink into its substance, to end in terminal arborisations around its cells. 
The corpus geniculatum externum receives the largest contribution of fibres from the 
lateral root of the optic tract. These partly sink into its interior and partly spread 
out over its surface. The former enter into the construction of the curved lamelle 
of white matter which traverse this nuclear mass, and to a large extent end in the 
gray matter which intervenes between these lamellee. The deep fibres which are 
not exhausted in this way proceed onwards through the external geniculate body 
and enter the pulvinar. Of the superficial fibres which spread over the surface of 
the external geniculate body some dip into its substance and end there, but the 
majority are carried over it and enter the stratum zonale of the pulvinar. The 
fibres of the lateral root of the optic tract, which end in the pulvinar, therefore 
reach their destination by passmg either over or through the external geniculate body. 
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