II20 



HUMAN ANATOMY 



nucleus, and establishes the important pathway transmitting the fibre-tracts con- 

 necting the cerebral cortex with the thalamus and with the lower levels by way 

 of the crusta of the cerebral peduncle. Since the innumerable fibres which pass 

 to and from the thalamus along its ventro-lateral surface interlace, this 'surface is 

 covered by a distinct reticulated stratum, to which the name external medullary 

 lamina is applied. The ventral surface is also attached, but instead of being 

 united with the internal capsule, as is the lateral, it rests upon and is intimately 

 blended with the upward prolongation of the tegmental portion of the cerebral 

 peduncle, here known as the subthalamic tegmental region, presently to be 

 described (page 1127). 



Fig. 967. 



Corpus callosum 



Choroid plexus 



Fornix 



Taenia thalami 



Middle commissure 



Third ventricle 



Mammillo- thalamic 



tract 



Mamraillary body 



Amygdaloid 

 nucleu5 



Caudate nucleus 



Thalamus, 

 mesial nucleus 



Thalamus, 

 lateral nucleus 



Lenticular nucleus 



Subthalamic 

 nucleus 



Optic tract 



Tail of 



caudate nucleua 

 Inferior horn of 

 lateral ventricle 



Hippocampus, cut obliquely 

 Crusta of cerebral peduncle 



Frontal section of brain passing through thalami, middle commissure and mammillary bodies. 



Structure of the Thalamus. — Although composed chiefly of gray matter, 

 the thalamus is partially surrounded and penetrated by tracts of white matter. In 

 addition to being invested on its superior and ventro-lateral surfaces by the stratum 

 zonale and the external medullary lamina respectively, the general ganglionic mass 

 is subdivided by a vertical internal sheet of fibres, continuous with the stratum zonale 

 and known as the internal medullary lamina, into three fairly marked nuclei, 

 the anterior, the mesial and the lateral (Fig. 967). Of these the lateral nucleus is 

 much the largest and is included between the external and internal medullary laminae. 

 Whilst the lateral nucleus does not reach as far forward as the anterior pole of the 

 thalamus, its caudal extremity includes the entire pulvinar. The lateral nucleus 

 consists histologically of an intricate complex of nerve-fibres and cells. The latter 

 are in general of the multipolar type, although very variable as to details of form 

 and size. Two principal types are recognized by Kolliker, the one being elongated 

 or fusiform and possessed of relatively few branches, and the other being stellate and 

 provided with richly branched dendrites. Many of the fibres represent paths ending 

 within the thalamus and therefore terminate in arborizations around the thalamic 

 cells ; others are the axones of such cells and ])ass to various parts of the cortex or 

 other parts of the brain. The histological characteristics of the lateral nucleus, in the- 



