PATHS FROM AND TO THE CORTEX 853 



of the body) of those muscles which -are accustomed to work with 

 the corresponding muscles on the opposite side e.g., the respiratory 

 muscles, these being innervated to some extent from both cerebral 

 hemispheres. 



(2) A great afferent or sensory path by which some at least of the 

 impulses carried up through the posterior roots of the spinal nerves, 

 after passing through various relays of nerve-cells, reach the cortex 

 of the cerebellum ; or the upper portions of the central grey tube, the 

 corpora quadrigemina and optic thalamus; or, finally (through the 

 tegmentum and the posterior limb of the internal capsule behind the 

 motor fibres), the cerebral cortex itself. 



The efferent pyramidal path from the cortex to the periphery is 

 broken by at most two relays of nerve-cells those intercalated cells 

 to which reference has already been made (p. 848), if they really 

 exist, and the motor cells of the anterior horn. The afferent path to 

 the cerebral cortex is interrupted by at least three relays with axons 

 of considerable length. One of the cells is situated in the ganglion 

 on the posterior root, another in the medulla oblongata, a third in the 

 optic thalamus ; and on some of the routes another, or even more than 

 one, is intercalated between the medulla and the cortex (Fig. 347). 



The Internal Capsule. We have already recognized the pyramidal 

 tract and the afferent tegmental path as constituents of the internal 

 capsule. The cranial fibres of the pyramidal tract occupy mainly 

 the genu or knee, the spinal fibres the posterior limb as far back as 

 the posterior border of the lenticular nucleus (Fig. 346). 



The fibres from the various motor areas are to a certain extent 

 arranged in order in the capsule, those for the eyes and head lying 

 farthest forward, those for the leg farthest back, while the fibres 

 going to the face, arm and trunk occupy intermediate positions. 

 The separation, however, is far from complete, the fibres of neigh- 

 bouring regions being considerably intermixed (Hoche). As the 

 tracts pass downwards the intermingling becomes continually 

 greater (Simpson and Jolly) (Figs. 348, 349). The afferent fibres 

 from the thalamus to the cortex, which We have described as the 

 last segment of the afferent tegmental path, lie in the posterior part 

 of the posterior limb. But here again there is no absolutely sharp 

 line of demarcation. Some motor fibres are intermingled with the 

 sensory in the posterior part of the capsule, for lesions of this region 

 produce a certain degree of paralysis as well as anaesthesia on the 

 opposite side of the body. A pure capsular hemiansesthesia that 

 is, a loss of sensation on the opposite side due to a lesion in the 

 internal capsule and unaccompanied by motor defect does not 

 appear to exist. Accordingly the common statement that the efferent 

 (motor) path occupies the anterior two-thirds, and the afferent 

 (sensory) path the posterior third of the posterior limb of the 

 internal capsule, while true in a general sense, is not strictly correct. 



