FUNCTIONS OF THE INTERNAL CAPSULE. 485 



to act as an intermediate between emotional states and their expres- 

 sion in the muscles of the face, this power being lost in certain patho- 

 logic conditions. The power of regulating the temperature of the 

 body has also been assigned to the thalamus, as destruction of its 

 anterior extremity is usually followed by a rise in temperature. 



The Internal Capsule. The internal capsule has been shown by 

 the results both of experiment and of pathologic processes to be, first, 

 a pathway for the transmission of nerve impulses from the cerebral 

 cortex to the pons, medulla, and spinal cord, which give rise to 

 contraction of the muscles of the opposite side of the body; and, 

 second, a pathway for the transmission of nerve impulses coming 

 from skin, mucous membrane, muscles, and special 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 important parts of the central 

 nerve system. As shown in Fig. 218, 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 

 cpntains 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. 218. 



The relation of the internal capsule to the caudate nucleus and 

 the optic thalamus internally, and to the lenticular nucleus exter- 

 nally, is also shown in a vertical section of the cerebrum made in 

 front of the gray commissure (Fig. 219). 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. 



