_Disturbances of co-ordination_ may be demonstrated by telling the patient to touch the tip of his nose with his finger, or to strike at an object held a foot or two in front of him. Such attempts are characterized by uncertainty and irregularity of movement, accompanied by considerable tremor.
_Paresis or paralysis of the limbs on the ipso-lateral side._ The abscess usually occupies the antero-external aspect of the lateral lobe of the cerebellum, and is so situated that no direct pressure is exercised on the pyramidal fibres. Some weakness--perhaps paralysis--of the muscles of the extremities on the same side as the lesion can, however, usually be demonstrated. According to Luciani, this is explained in the following manner:--between the cortex of the one side and the cerebellum of the other there are certain ‘associated’ fibres, the strength of the impulses sent out from the cortex depending in part on the integrity of these reinforcing cerebellar fibres. In cases of cerebellar abscess these reinforcements are cut off with consequent paresis, or even paralysis, of the muscles of the face, arm, and leg on the same side as the lesion. Associated with this muscular weakness there is usually some increase in the deep reflexes. Some of the pyramidal fibres do not decussate, and, consequently, an ipso-lateral paralysis may be associated with a contra-lateral paresis.
The same lessening of cortical impulses accounts for a weakening in the external rectus of the same side, which muscle, acting with the internal rectus of the sound side, allows of a conjugate deviation of the eyes towards the opposite side, with well-marked _nystagmus_--of a coarse type--on attempting to correct this deviation.
[Illustration: FIG. 78. A DIAGRAMMATIC ILLUSTRATION (AFTER LUCIANI) TO EXPLAIN THE SYMPTOMS OBSERVED IN CEREBELLAR ABSCESS FORMATION. P., Pyramidal fibres; C.A., Right cerebellar abscess; R., Reinforcing fibres from right cerebellum to left cortex; B.g., Basal ganglia; E.R., External rectus; N., Nucleus for sixth nerve; D.P.T., Direct pyramidal tract; C.P.T., Crossed pyramidal tract.]
_Retraction of the head and neck._ Retraction of the head and neck, stiffness of the nuchal muscles, and even opisthotonos, may be observed. The existence of these symptoms is always suggestive of meningeal infection, but an abscess of considerable size, even when situated in the anterior part of the lateral lobe of the cerebellum, may so exercise pressure in the downward direction as to cork up that part of the cerebellum which normally extends into the mouth of the foramen magnum. The neighbouring upper cervical nerves may then be irritated or compressed.