676 THE NERVOUS SYSTEM. 



gemina, with which they come in close contact. They then pass under these 

 bodies, through the tegmentum (vide supra), and enter the optic thalamus. 



The corpora geniculata are two small, oblong masses on each side, situated 

 behind and beneath the posterior end of the optic thalamus, and named, from their 

 position, corpus geniculatum externum and internum. These two bodies are sepa- 

 rated from each other by the brachium anterius of the anterior quadrigeminal body. 

 It is convenient and customary to describe these two bodies together, but the 

 student should bear in mind that the corpus geniculatum externum belongs in 

 reality to the optic thalamus ; the corpus geniculatum internum alone being a part 

 of the mid-brain. The external geniculate body is of a dark color, and presents a 

 laminated arrangement, consisting of alternate layers of gray and white matter. 

 Its cells are large, multipolar, and pigrnented ; their processes are intimately related 

 with the visual area in the cerebral cortex of the occipital region. It is believed 

 that the intercellular gray matter of these bodies is composed, to a considerable 

 extent, of the terminations of the optic nerve, which form synapses around the cells. 

 The internal geniculate body is smaller in size, lighter in color, and does not pre- 

 sent a laminated arrangement. It receives the posterior brachium from the inferior 

 quadrigeminal body, and some of the fibres of the optic tract appear to enter it. 

 The internal geniculate bodies are connected with each other through the optic 

 commissure by a band of fibres named Gruddens commissure (see page 721). The 

 anterior quadrigeminal body, the pulvinar, and the external geniculate body are 

 intimately concerned with vision. They constitute the lower cerebral centre for 

 the optic nerve-fibres which end in them. Extirpation of the eyes in newly born 

 animals entails an arrest of their development, but has no effect on the poste- 

 rior quadrigeminal body or the internal geniculate body. These latter also are 

 well developed in the mole, where the superior quadrigeminal body is rudi- 

 mentary. 



The Aqueduct of Sylvius, or Iter a Tertio ad Quartum Ventriculum. This is a 

 narrow canal, about half an inch in length, situated between the corpora quadri- 

 gemina and the tegmentum, and connecting the third with the fourth ventricle. 

 Its shape on transverse section varies, being T-shaped below, triangular above, and 

 oval about the middle of its course. It is lined by columnar ciliated epithelium, 

 and surrounded by a layer of gray matter, called the central gray matter of the 

 aqueduct, which is continuous with the gray matter of the third and fourth ven- 

 tricles. This gray matter is separated above from that of the corpora quadri- 

 gemina by the stratum lemnisci ; below it, is the posterior longitudinal bundle and 

 the formatio reticularis of the tegmentum. The central gray matter is more 

 abundant below the canal than above it. Here are certain defined group of 

 cells, which are connected with the roots of the third, fourth, and fifth cranial 

 nerves. 



Subthalamic Region. One other structure, to which allusion has already been 

 made, requires mention in this connection ; it is the subthalamic region. It is a 

 prolongation forward of the tegmentum of the crus cerebri, which becomes con- 

 tinuous with the lower surface of the optic thalamus. Toward the anterior part 

 of the crus cerebri the tegmentum becomes thinned out, and is blended with the 

 superjacent portion of the optic thalamus. To this region, the name subthalamic 

 tegmental region has been given. In front it is lost at the base of the brain in 

 the gray matter of the anterior perforated space, and is continuous with the gray 

 matter of the floor of the third ventricle. The subthalamic tegmental region con- 

 tains a forward prolongation of the red nucleus, and consists from above downward 

 of three layers : (1) stratum dorsale, which is directly applied to the under surface 

 of the optic thalamus, and consists of fine longitudinal fibres ; (2) zona incerta, a 

 continuation forward of the formatio reticularis of the tegmentum ; and (3) the 

 corpus subthalamicum, a mass of gray matter which on -section presents a lenticular 

 shape, and lies immediately above the substantia nigra. 



