536 TEXT-BOOK OF PHYSIOLOGY. 



sense-organs to the cortex, where they give rise to sensations general and 

 special. It is therefore the common motor and sensor pathway. For the 

 reason that it transmits both motor and sensor impulses, and for the further 

 reason that it is frequently the seat of pathologic lesions which are followed 

 by either a loss of motion or sensation or both, the internal capsule is one 

 of the most interesting parts of the central nerve system. As shown in Fig. 

 246, it consists of two segments or limbs united at an obtuse angle, the knee 

 or elbow, which is directed toward the median line. The motor tract is 

 confined to the posterior one-third of the anterior segment and the anterior 

 two-thirds of the posterior segment. The sensor tract is confined to the 

 posterior one-third of the posterior segment, the extreme end of which also 

 contains the optic and auditory tracts. 



The region of the anterior segment in front of the motor tract contains 

 the fibers of the fronto-cerebellar tract, the function of which is unknown. 



The motor region contains fibers which descend from the cerebral cortex 

 to nerve-centers situated in the gray matter beneath the aqueduct of Sylvius, 

 in the gray matter beneath the floor of the fourth ventricle, and in the anterior 

 horns of the gray matter of the spinal cord, and which in turn are connected 

 by the cranial and spinal nerves with the muscles of the eye, head, face, 

 trunk, and limbs. The positions occupied by these different tracts are 

 shown in Fig. 246. 



The relation of the internal capsule to the caudate nucleus and the optic 

 thalamus internally, and to the lenticular nucleus externally, is also shown 

 in a vertical section of the cerebrum made in front of the gray commissure 

 (Fig. 237). From the fact that the internal capsule contains efferent or 

 motor tracts, and afferent or sensor tracts, it is evident that a destructive 

 lesion of the motor tract would be followed by a loss of motion; and of the 

 sensor tract, by a loss of sensation on the opposite side of the body. 



