THE SPINAL AXIS. 291 



CHAPTER XV. 



THE SPINAL AXIS. 



Primitive Development of Nervous System. Itsjinal Condition in different Vertebrates. 



The Spinal Cord: its Structure. Its Membranes. Its Thirty-one Pairs of Nerves. Proper- 

 ties of their Hoots. Functions of the Cord. Bell's Discovery. Transmission of Longitudinal 

 and Transverse Influences. Reflex Action of the Cord. Nature of Reflex Action. Motor and 

 Sensory Tracts of the Cord. Summary of its Functions.. 



The Medulla Oblongata : its Structure and Functions. 



The Pons Varolii: its Structure and Functions. 



Dr. Carpenter's Views of the Analogy between tlie Spinal Cord of Vertebrates and the Ventral 

 Cord of Articulates. 



WE now commence a more detailed examination of the nervous sys- 

 tem, presenting a description of its structure as far as may Subdivisions 

 be necessary for the understanding of its functions. We of the sub J* ect - 

 shall follow the usual division of this subject as adopted by authors. 

 This will therefore lead us to speak in succession of the spinal cord and 

 medulla oblongata, of the sensory ganglia, of the cerebellum and cere*- 

 brum, of the nerves generally, and, lastly, of the sympathetic system. 



The important position occupied by the nervous mechanism in the an- 

 imal body will always draw to it the closest attention of the physiol- 

 ogist, and yet it must be admitted that hitherto it is the least ad- 

 vanced portion of the science. If metaphysicians are to be blamed for 

 casting away the advantages which arise from a study of Advantages de- 

 structure, the earlier physiologists were almost equally in 

 error in confining themselves to human anatomy alone, 

 They did this under an impression that there is an essential and intrinsic 

 difference between the functions of this system in man and in the lower 

 animals. 



There is an analogy of construction in all the forms of nervous system 

 presented by the different animal tribes, which, in the infancy of the sci- 

 ences of organization, was attributed to a unity of design pervading the 

 plan of Nature, but which, when seen from a higher and more philosoph- 

 ical point of view, is plainly the necessary result of a universal and un- 

 varying law of development. This conclusion, which, when better un- 

 derstood, is doubtless destined to become one of the most important sug- 

 gestions ever furnished by science respecting the management of the 

 world, is strikingly enforced by the analogies between the Advant<> esde- 

 successive transitory stages of development of this system rived from de- 

 at different epochs in the life of man, and the permanent vel P ment - 

 form it assumes in members of the entire animal series. Since there can 



