This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The corona radiata is the bundle of white fibres which spreads out like a fan and connects the cortex of the brain with the basal ganglia and spinal cord. Proceeding downward from the cortex, the corona radiata becomes smaller and passes, in the form of a band, between the lenticular nucleus on the outside and the caudate nucleus and thalamus on the inside. This band is known as the internal capsule. It transmits in its anterior portion fibres from the prefrontal or higher psychical area; then come the motor paths; and still farther back, in the posterior third of the posterior portion, sensory fibres. The functions of the external capsule, which lies to the outer side of the lenticular nucleus, are not known.
Fig. 48. - Diagram showing course and decussation of corticospinal (pyramidal) tract; M, medulla; P,pons; CP, cerebral peduncle; T, thalamus; C, L, caudate and lenticular nuclei; CC, corpus callosum. (Piersol).
The motor fibres of the internal capsule pass downward through the anterior portion of the crus cerebri and pons into the medulla, at the lower part of which the majority decussate and pass into the anterior columns of the cord as the pyramidal tracts. Thus, it is seen that destruction of any portion of the motor tract, from the point of pyramidal decussation below, through the internal capsule to the cortex above, will cause a paralysis on the opposite side of the body.