268 SPECIAL PHYSIOLOGY. 



the third being higher up, some of its fibres being even connected with 

 the corpora quadrigemina. Below the sensory ganglion of the fifth, 

 is that of the glosso-pharyngeal, the two masses touching and blend- 

 ing, and suggesting an explanation of the common gustatory function 

 of certain branches of these two nerves ; below this, is the sensory 

 ganglion of the vagus, which lies immediately above the upper end of 

 the posterior gray matter of the cord. In front of these sensory 

 ganglia, and nearer the centre of the medulla, are the ganglia of origin 

 of the motor cranial nerves ; that of the motor root of the fifth being 

 found opposite the upper part of the medulla, in close proximity with 

 that of the sixth, and the portio dura or motor part of the seventh, 

 which lie below and a little behind it ; whilst the ganglia of the hypo- 

 glossal and spinal accessory nerves are found a little lower down, nearer 

 to the centre of the medulla, and above, and in a line with, the anterior 

 gray masses of the spinal cord. 



Each of these ganglionic centres, is independent as to its proper 

 functions, as may be shown, in some cases, by artificial sections, or 

 by the effects of disease; but, during life, they are all associated, and 

 are subordinated to the action of the cerebrum. 



On comparing the cranial nerves with a typical spinal nerve, certain 

 homologies are evident. The sensory portion of the fifth pair, to- 

 gether with the motor nerves of the orbit, viz., the third, fourth, and 

 sixth nerves, and, lastly, its own motor portion, constitute a compound 

 cranial nerve, homologous with a spinal nerve; the glosso-pharyngeal 

 with the facial constitute a second; and the pneumogastric and spinal 

 accessory form a third compound nerve, like a spinal nerve; the hypo- 

 glossal is, according to this view, supposed to be a spinal nerve, some- 

 times having in certain animals, as already mentioned, a small sensory 

 root. The three complete compound cranial nerves just described, 

 pass out of the cranium between its component segments, as the spinal 

 nerves emerge through the intervertebral foramina. Of the nerves of 

 special sense, the olfactory, optic, and auditory penetrate certain 

 cranial elements, passing respectively through, or into, the ethmoid, 

 sphenoid, and temporal bones. The optic and olfactory tracts are, 

 indeed, not nerves, but processes of the cerebrum. 



Functions of the Spinal Cord. 



The functions of the spinal cord have, in the first place, to be con- 

 sidered in reference to sensation and to voluntary motion ; and after- 

 wards, in regard to the regulation of those peculiar movements, which 

 constitute the so-called reflex action. 



As regards sensation, the spinal cord, like the nerves, is only a con- 

 ductor of the effects of sensory impressions; for, when it is divided, 

 compressed, or otherwise injured or diseased at any part of its course, 

 even up to the medulla, all the parts, supplied by nerves arising from 

 it below the seat of injury, lose their sensibility, no pain being pro- 

 duced by pinching or otherwise irritating them. Neither is the cord 

 itself sensitive ; for, when it is divided, irritation of its lower portion 

 produces no pain ; but irritation of the upper portion of the cord, still 



