666 NERVOUS SYSTEM. 



CHAPTER XX. 



FUNCTIONS OF THE SPINAL CORD. 



General arrangement of the cerebro-spinal axis Membranes of the encephalon and spinal cord Cephalo-rachidian 

 fluid Physiological anatomy of the spinal cord Direction of the fibres after they have penetrated the cord by the 

 roots of the spinal nerves General properties of the spinal cord Action of the spinal cord as a conductor Trans- 

 mission of motor stimulus in the cord Decussation of the motor conductors of the cord Transmission of sen- 

 sory impressions in the cord The white substance of the posterior columns does not conduct sensory impres- 

 sionsAction of the gray matter as a conductor Probable function of the cord in connection with muscular co- 

 ordination Decussation of the sensory conductors of the cord Summary of the action of the cord as a conductor 

 Action of the spinal cord as a nerve-centre Movements in decapitated animals Definition and applications 

 of the term u reflex " Reflex action of the spinal cord Question of sensation and volition in frogs after decapita- 

 tion Character of movements following irritation of the surface in decapitated animals Dispersion of impres- 

 sions in the cord Conditions essential to the manifestation of reflex phenomena Exaggeration of reflex excita- 

 bility by decapitation, poisoning with strychnine, etc. Eeflex phenomena observed in the human subject. 



UNDER the head of special senses, we shall consider, in succeeding chapters, the prop- 

 erties and functions of the first and second nerves, the portio mollis of the seventh, or 

 auditory, and the gustatory nerves, comprising a part of the glosso-pharyngeal and a 

 small filament from the facial (the chorda tympani) going to the lingual branch of the 

 fifth. This will include a full account of the organs of smell, taste, sight, and hearing, 

 with a description of the general sensory nerves, as far as they are concerned in the sense 

 of touch. We shall here begin our history of the cerebro-spinal axis, which will include 

 the physiological anatomy, properties, and functions of the encephalon and the spinal cord. 



General Arrangement of tlte Cerebro-spinal Axis. The nervous matter contained in 

 the cavity of the cranium and in the spinal canal, exclusive of the roots of the cranial 

 and spinal nerves, is known as the cerebro-spinal axis. This portion of the nervous sys- 

 tem is composed of white and gray nervous matter. The fibres of the white matter act 

 as conductors. The gray matter constitutes a chain of ganglia, which act as nerve- 

 centres, receiving impressions and generating the so-called nerve-force. The gray matter 

 of the spinal cord also serves, to a greater or less extent, as a conductor. 



The cerebro-spinal axis is enveloped in membranes, which are for its protection and 

 for the support of its nutrient vessels. It is surrounded, to a certain extent, with liquid, 

 and it presents cavities, as the ventricles of the brain and the central canal of the chord, 

 which contain liquid. The gray matter is distinct from the white, even to the naked 

 eye. In the spinal cord, the white substance is external and the gray is internal. The 

 surface of the brain presents an external layer of gray matter, the white substance being 

 internal. In the white substance of the brain, also, we find collections of gray matter. 

 As we should expect, from the similarity in function between the white matter and the 

 nerves, this portion of the cebro-spinal axis is composed largely of fibres. The gray 

 substance is composed chiefly of cells. 



The encephalon is contained in the cranial cavity. In the human subject and in 

 many of the higher animals, its surface is marked by numerous convolutions, by which 

 the extent of its gray substance is very much increased. The cerebrum, the cerebellum, 

 and all of the encephalic ganglia are connected with the white substance of the encepha- 

 lon and with the spinal cord. With the encephalon and the cord, all of the cerebro- 

 spinal nerves are connected. The cerebro-spinal axis acts as a conductor, and its diifer- 

 ent collections of gray matter, or ganglia, receive impressions conveyed by the sensory 

 conducting fibres, and generate nerve-force, which is transmitted to the proper organs 

 by the motor fibres. 



Membranes of the EmepJialon and Spinal Cord. The membranes of the brain and 

 spinal cord are, the dura mater, the arachnoid, and the pia mater. 



