THEOMBOSIS OF CEEBBEAL SINUSES. 173 



not favour embolism as has previously been explained. Since 

 the occlusion is not at first complete, remissions and exacerba- 

 tions of symptoms may be expected, and these fluctuations 

 in the amount and distribution of paralysis may help in the 

 distinction of thrombotic from embolic softening. The lower 

 part of the basilar artery is held to supply the median or 

 chief branches to the vagus centre ; its upper part is directly 

 continuous with the posterior cerebral trunks into which the 

 clot may extend. If unilateral softening occur in the pons above 

 the decussation of the facial fibres the resulting hemiplegia will 

 not differ from the capsular type ; face, arm and leg all being 

 affected on the side opposite to the lesion. If the softening be in 

 the lower half of the pons, an alternate, or crossed, paralysis may 

 occur and the face suffer on the same side as the lesion, whilst 

 the limbs are paralysed on the opposite side. Of course both 

 halves of the pons may become softened and so produce bilateral 

 paralysis. The sixth nerve nucleus is some distance below the 

 third and fourth nuclei, so in lesions of the lower half of the pons 

 the sixth may be paralysed on the same side as the facial nerve, 

 i.e., opposite to the limbs. As the sixth nucleus is the centre for 

 conjugate deviation to its own side the patient will look away 

 from the lesion owing to the unantagonised action of the muscles 

 of the other side. 



The vertebral artery is a variable vessel ; the left is usually 

 larger than the right, indeed the basilar artery may appear to be 

 chiefly formed by the left vertebral. The vertebral is responsible, 

 near its termination, for the blood supply of the hypoglossal and 

 spinal accessory nuclei, and its occlusion produces unilateral labio- 

 glosso-laryngeal paralysis on the side of the. lesion as well as 

 interference with the motor and sensory tracts of the medulla. 



THROMBOSIS OF CEREBRAL SINUSES. 



Thrombosis of the cerebral blood sinuses may be primary, 

 traumatic, or inflammatory. Primary thrombosis is met with 

 in certain debilitating conditions at the two extremes of life. 



