2l6 THE NERVOUS SYSTEM 



from each other by the cleft-like space of the third ventricle, and are united across 

 the median plane by a short bar of gray substance, the massa intermedia. 



The lateral surface is hidden from view. It lies against the broad band of 

 fibers, known as the internal capsule, which connects the cerebral hemispheres 

 with the lower levels of the central nervous system. This surface is best examined 

 in sections through the entire cerebrum (Figs. 155-157). Many fibers stream 

 out of the thalamus through its lateral surface and enter the internal capsule, 

 through which they reach the cerebral cortex. To this important stream of 

 fibers the name thalamic radiation is applied. 



The ventral surface of the thalamus is also covered from view and lies on the 

 hypothalamus, by which it is separated from the tegmentum of the mesencepha- 

 lon (Figs. 155, 157). Many fibers, representing such ascending tegmental paths 

 as the medial lemniscus, spinothalamic tract, and brachium conjunctivum, enter 

 the thalamus through this surface. 



Structure of the Thalamus. The thalamus consists chiefly of gray matter, 

 within which there may be recognized a number of nuclear masses. Its dorsal 

 surface is covered by a thin layer of white matter, called the stratum zonale, 

 which in the region of the pulvinar consists in large part of fibers derived from 

 the optic tract. On the lateral surface of the thalamus next the internal cap- 

 sule there are many myelinated fibers, which constitute the external medullary 

 lamina (Figs. 155, 156). The medial surface is covered by a layer of central 

 gray matter, continuous with that which lines the cerebral aqueduct and forms 

 the floor of the third ventricle. This central gray matter consists of neuroglia 

 and of scattered nerve-fibers and cells (the nucleus paramedianus of Malone, 

 1910). Some of these fibers are continued through the gray matter that lines 

 the aqueduct and the floor of the fourth ventricle, as the dorsal longitudinal 

 bundle of Schutz (Fig. 112). It is probable that this portion of the thalamus 

 forms a center for vasomotor and visceral reflexes, since lesions in this region 

 are often accompanied by disturbances in the nervous control of the blood- 

 vessels and viscera (Edinger, 1911; Rogers, 1916). If this be true, it is probable 

 that the dorsal longitudinal bundle of Schutz serves to bring this thalamic 

 mechanism for visceral adjustments into connection with the visceral efferent 

 nuclei of the brain. 



From the stratum zonale, which clothes its dorsal surface, there penetrates 

 into the thalamus a vertical plate of white matter, called the internal medullary 

 lamina. This subdivides the thalamus into three parts: the anterior, medial, 

 and lateral nuclei. At the rostral extremity of its dorsal border the internal 



