920 THE CENTRAL NERVOUS SYSTEM 



performed by section of all the respiratory nerves at a distance from 

 the medulla oblongata. But while nobody speaks of the destruction of 

 a ' centre ' when a reflex action is abolished by division of the peripheral 

 nerves concerned in it, there is a tendency, when the same effect is 

 brought about by a lesion in the brain or cord, to invoke that mysterious 

 name, and to forget that the cerebro-spinal axis is at least as much a 

 stretch of conducting paths as a collection of discharging nervous 

 mechanisms. 



It is, perhaps, a profitless task to enumerate all the so-called centres 

 in the bulb and cord. In addition to the great vaso-motor, respiratory, 

 cardio-inhibitory and cardio-augmentor centres in the bulb, which 

 perhaps, have more right than the rest to be regarded as distinct 

 physiological mechanisms, if not as definitely bounded anatomical 

 areas, there have been distinguished ano-spinal, vesico-spinal, and 

 genito-spinal centres in the lumbar cord, a cilio-spinal centre for dila- 

 tation of the pupil in the cervical cord, and in the medulla centres for 

 sneezing, for coughing, for sweating, for sucking, for masticating, for 

 swallowing, for salivating, for vomiting, for the production of general 

 convulsions, for closure of the eyes, for the secretion of tears, and even a 

 ' diabetes ' or ' sugar ' regulating centre (p. 547). It has been recently 

 shown that in the cat a region of the dorsal cord between the last 

 cervical and the third dorsal segments is capable of sustaining the 

 spontaneous liberation of epinephrin from the adrenal glands after the 

 cord has been divided higher up. 



SECTION IX. THE CRANIAL NERVES. 



Unlike the spinal nerves, which arise at not very unequal intervals 

 from the cord, the nuclei of the cranial nerves, with the exception 

 of the olfactory and optic, are crowded together in the inch or two 

 of grey matter of the primitive neural axis in the immediate neigh- 

 bourhood of the fourth ventricle and the Sylvian aqueduct. Of 

 these nuclei some are the end nuclei or ' nuclei of reception ' of 

 sensory fibres that is to say, collections of nerve-cells around 

 which the sensory fibres break up into terminal arborizations. Such 

 are the sensory nuclei of the fifth, the nuclei of the eighth, and the 

 sensory nuclei of the glossopharyngeal and vagus nerves (Figs. 367, 

 368). The nuclei of origin of the motor fibres lie, upon the whole, 

 in two longitudinal rows a. median row, which consists of the 

 nuclei of the third and fourth nerves in the floor of the aqueduct, 

 and those of the sixth and twelfth nerves in the floor of the fourth 

 ventricle ; and a lateral row comprising the motor nuclei of the fifth, 

 seventh, tenth, and eleventh nerves. The clumps of grey matter 

 which make up these nuclei may be considered as homologous with 

 the grey matter of the ventral or anterior (including the lateral) 

 horn of the spinal cord ; and the motor fibres of the nerves themselves 

 as homologous with the anterior spinal roots. Without going 

 further into the thorny subject of the homologies of the cranial and 

 spinal nerves, we may point out that while all the spinal nciv 

 contain both efferent and afferent fibres, some of the cranial neiv 



