LONG TRACTS.
The fibres pass from the cortex through the internal capsule to the crusta or basis pedunculi cerebri, to end within the pons, the medulla oblongata and the spinal cord. The chief tracts are :— 1. The frontal pantile tract. The fibres arise within the cortex of the fron tal lobe, traverse the posterior part of the anterior limb of the internal capsule form the inner fifth of the basis pedunduli and end within the pons in the pon tile nucleus.
2. The pontile tract. The fibres arise within the cortex of the occipital and temporal lobes, traverse the posterior segment of the internal capsule, form the outer fifth of the basis pedunculi and end within the pons in the pontile nucleus. The tractus corticis ad pontem further is joined by the tractus con necting the pons with the cerebellum (Figs. 130 and 133).
3. The motor tract. The fibres arise within the cortex of the precentral convolution and the paracentral lobule, pass through the knee and anterior two-thirds of the posterior limb of the internal capsule, form the middle three fifths of the basis .pedunculi, and continue to the medulla oblongata and the spinal cord. The entire motor tract comprises the and tracts (Fig. 132).
a. The tract or tract of the motor cerebral nerves. The origin of the fibres is known for only the facial and hypoglossal nerves, the fibres of which arise within the cortex of the lower part of the precentral convolution. The tract passes through the knee of the internal capsule to the basis pedunculi cerebri and ends in the nuclei of the motor nerves of the opposite side.
b. The tract or tract of the motor spinal nerves. The fibres of this path, also known as the tractus or the pyramidal tract, take origin in the cortex of the lobulus paracentralis and of the upper and middle parts of the motor region of the precentral convolution, traverse the anterior two-thirds of the posterior • limb of the internal capsule, and continue through the basis pedunculi and the pons to the medulla oblongata. At the transition of the medulla to the spinal cord, the fibres of the pyramidal tract cross to the opposite side, forming the pyramidal decussation. The latter, however, is not complete, since a small portion of the fibres continues uncrossed in the anterior column of the spinal cord as the fasciculus anterior or anterior pyramidal tract. The termination of these fibres is within the anterior horn of the spinal cord and, moreover, in the anterior cornu of the opposite side, the fibres crossing through the anterior commissure. The larger part of the fibres crosses to the oppo site side, and descends in the lateral column of the spinal cord as the fasciculus cerebro spinalis lateralis or lateral pyramidal tract, to end in the anterior horn of the same side.
The course of the motor tract explains the fact, that movements induced by stimulation of the motor cortical region occur chiefly in the muscles of the opposite half of the body, or that injury of the central neurones of the motor tract is followed by paralysis of the muscles of the opposite half of the body. Such paralyses of one side (hemiplegia) are usually caused by lesions within the capsula interna, less frequently by lesions within the cerebral peduncle or the pons. Since
the speech-tract takes its origin within the left hemisphere, lesions of the motor paths within the left hemisphere, or right-sided hemiplegias, are usually associated with disturbances of speech.
In Fig. 136, the course of the motor tract is schematically represented to explain the most important forms of paralysis. In total hemiplegia, hemiplegia completa (Fig. 136, a), the destruction of an entire descending motor tract from one hemisphere is concerned. In such cases, the lesion usually lies within one motor tract somewhere along the brainstem between the internal capsule and the pyramidal decussation in the medulla oblongata, since within this stretch all the descending motor fibres are com pressed into a field of small area. Most frequently the lesion is situated within the internal capsule (knee and anterior two-thirds of the posterior limb), less fre quently within the cerebral peduncle and the pons. If in a lesion of the motor tract within the internal capsule the knee of the capsule remains uninvolved, the facial and hypoglossal nerves do not share in the paralysis, such condition con stituting hemiplegia incompleta (Fig. 136, b) . In case the lesion be located within the region of the cerebral peduncle, the emerging fibres of the oculomotor nerve are often also implicated. Under such conditions a homolateral oculomotor paralysis exists in conjunction with the crossed hemiplegia, the condition being designated as hemiplegia alternans oculomotoria, or Weber' s paralysis ( Fig. 136, c). Hemiplegia alternans is also encountered in affections of the pons and in lesions within the range of the medulla oblongata. Thus, in pontile lesion, paralysis of the extremities on one side occurs with paralysis of the facial nerve on the other—hemiplegia alternans facialis or Gubler' s paralysis ( Fig. 136, d). Further combinations are : crossed limb-palsy with homolateral paralysis of the abducens, or crossed hemiplegia with homolateral hypoglossal or lingual paralysis.
Hem iplegias following complete destruction of the entire motor cortical region of one hemisphere are rare, by reason of the large extent of the motor centre. Cortical diseases are more frequently limited to circumscribed areas and paralyses resulting from cortical lesion are confined, as a rule, to particular portions of one-half of the body. In such cases one speaks of monoplegia, or, more definitely, as monoplegia cruralis, monoplegia brachialis, or monoplegia facialis, according to the involvement of the motor centre for the leg, arm, or face respectively. Such palsies are frequently associated with sudden seizures of convulsions ( cortical or Jacksonian epilepsy).
A lesion of both pyramidal tracts descending in the anterior and lateral columns of the spinal cord leads to paraplegia or paralysis of both upper or lower extremities (Fig. 136, f, brachialis or superior and paraplegia cruralis or inferior. In very rare cases, the lesion may involve the pyramidal decussation in such manner, that the fibres for one extremity are interrupted above and those for the other below their place of crossing. In such cases hemiplegia cruciata results, that is, paralysis of the arm on one side and of the leg on the other (Fig. 136, e).