CENTRES OF SENSATION. 267 



and sterno-mastoid ; but if the roots of the vagus are first divided, only 

 the two last-named muscles contract. This is explained by supposing 

 that in the former case the movements are reflex, and excited through 

 afferent fibres contained in the spinal accessory, which convey the 

 stimulus to the medulla oblongata, and thence along the vagus nerve. 

 Section of the muscular branches of the spinal accessory, only par- 

 tially paralyzes the trapezius and the sterno-mastoid, because both 

 these muscles also receive branches from spinal nerves. 



The ninth, last cranial, or hypoglossal nerve, is purely motor, re- 

 ceiving, however, a few sensory fibres from the lingual branch of the 

 fifth, the vagus, and some of the cervical nerves. Pinching or galvan- 

 izing the nerve, at its point of exit from the cranium, causes violent 

 movements in the whole tongue ; section of the nerves paralyzes the 

 muscles of the tongue, without affecting either the common sensibility 

 or the gustatory sense. Irritation of the distal portion of the divided 

 nerve excites contractions in all the lingual muscles; whilst irritation 

 of the central portion still connected with the medulla oblongata, in- 

 duces signs of pain, owing to the admixture of sensory fibres from 

 other nerves, as above mentioned, external to the cranium. When 

 one hypoglossal nerve only, or its centre of origin, is affected by dis- 

 ease, the tongue is paralyzed on that side only, and when protruded, 

 the tip is generally turned towards the side on which the muscles are 

 paralyzed. Besides this, one proper muscle of the larynx, and certain 

 muscles of the neck which are connected with that organ, are para- 

 lyzed by section of this nerve, which, therefore, serves not only to con- 

 trol the tongue, but also to adjust the larynx in speech. 



Deep Connections of the Nerves and Centres of Sensation. On 

 tracing the roots of the several cranial nerves into the parts of the 

 encephalon from which they appear to spring, it is found that they all 

 have their origin in certain masses of gray matter^ which accordingly 

 are regarded as their proper ganglionic centres, and, in the case of the 

 special sensory nerves, are believed to be endowed with modified forms 

 of excitabjlity or sensibility. The so-named ganglia of origin of the 

 nerves of special sense, are very distinct, viz., the olfactory lobes, the 

 optic lobes, and the auditory and gustatory masses of gray matter at 

 the back of the medulla oblongata ; whilst the nerves of common sen- 

 sation and motion, cranial or spinal, arise from masses of gray matter, 

 all situated below the optic thalami and corpora striata. It will be 

 found, moreover, that the sensory nerves, or their sensory roots, orig- 

 inate from gray matter placed at the back of the medulla, in the course 

 of the great sensory tract of white fibres, and in a line with the pos- 

 terior portion of the gray matter, and the posterior columns of the 

 cord; whilst the motor nerves, or their motor roots, spring from other 

 masses of gray matter, associated with the great anterior or motor 

 tracts, which pass down from the cerebral peduncles to the anterior 

 and lateral columns of the medulla and cord. But the sensory gan- 

 glionic centres of the fifth nerves disappear altogether opposite the pons, 

 so that the two motor nerves above this point, viz., the third and 

 fourth, arise from gray matter placed close to the back of the upper 

 part of the cerebral peduncles, near the floor of the fourth, ventricle, 



