458 THE BRAIN 



Stretching across the interval between them, and bringing them into 

 continuity with one another, is a thin lamina called the anterior medullary 

 velum. When the brachia conjunctiva reach the bases of the inferior 

 quadrigeminal bodies, they sink into the substance of the mesencephalon, 

 and, in a transverse section through the lower part of that portion of the 

 brain, they may be seen as two white strands, semilunar in outline and 

 plaqed one on each side of the grey matter of the aqueduct. As they 

 ascend, they gradually assume a deeper (i.e. a more ventral) position in 

 the tegmental part of the mesencephalon, and they decussate with each 

 other across the median plane and proceed upwards to the red nuclei. 



The term lemniscus (O.T. fillet] is given to two tracts which have 

 different connections. The medial lemniscus (Figs. 180 and 181) is a 

 sensory tract passing upwards to the thalamus. The lateral lemniscus 

 belongs to the acoustic apparatus, and is a part of a chain through which 

 the cochlear. nuclei of one side establish connection with the inferior 

 quadrigeminal body and the medial geniculate body of the opposite side. 

 The lateral lemniscus can be readily detected as it emerges from the 

 upper part of the lateral sulcus of the mid-brain, and passes, backwards 

 and upwards, to the lower border of the inferior quadrigeminate body and 

 inferior brachium. It has the form of a raised triangular band which 

 encircles the lateral surface of the upper end of the brachium conjunctivum 

 (Fig. 194). 



Within the upper part of the tegmentum there is a collection of nuclear 

 matter which is termed the nucleus ruber, from its ruddy appearance 

 when seen in section. It is rod-like in form, and extends upwards into the 

 tegmental region below the thalamus (Fig. 188). In transverse section it 

 presents a circular outline, and it is closely associated with the upward 

 prolongations of the majority of the fibres of the brachium conjunctivum 

 of the opposite side. The brachium conjunctivum cerebelli is an efferent 

 tract from the nucleus dentatus of the hemisphere of the cerebellum, and 

 its fibres end in the red nucleus and the pulvinar of the thalamus of the 

 opposite side. The tegmentum of each pedunculus cerebri may be con- 

 sidered to consist of two parts : viz., a lower part, which is subjacent to 

 the inferior quadrigeminal bodies, and is largely occupied by the decussation 

 of the brachia conjunctiva cerebelli ; and an tipper part, subjacent to the 

 superior quadrigeminal bodies, which is traversed by the emerging bundles 

 of the third nerve, and contains the nucleus ruber. 



BASAL GANGLIA OF THE CEREBRAL 

 HEMISPHERES. 



The basal ganglia of the cerebral hemispheres must now 

 be examined. They are (i) the caudate and lentiform nuclei, 

 which, together, form the corpus striatum, (2) the claustrum, 

 and (3) the amygdaloid nucleus. At the same time the com- 

 position of the thalamus and the external and internal 

 capsules should be studied. 



Dissection. The right and left portions of what remains of 

 the cerebrum must be separated from one another, if that has 

 not already been done, by a median sagittal incision. Anteriorly, 

 the incision must pass between the columns of the fornix, and 



