72 NERVOUS SYSTEM. 



BOOK I. 



ANIMAL FUNCTIONS OR FUNCTIONS OF RELATION. 



THE functions of relation consist, first, of sensibility, and, secondly, 

 of muscular motion, including expression or language. They are all 

 subject to intermission, constituting sleep; a condition which has, con- 

 sequently, by many physiologists, been investigated under this class ; 

 but as the functions of reproduction are influenced by the same condi- 

 tion, the consideration of sleep will be deferred until the third class of 

 functions has received attention. 



CHAPTER I. 



SENSIBILITY. 



SENSIBILITY is the function by which an animal experiences feeling, or 

 has the perception of an impression. In its general acceptation, it 

 means the property possessed by living parts of receiving impressions, 

 whether the being exercising the property has consciousness of it or 

 not. To the first of these cases in which there is consciousness 

 Bichat gave the epithet animal; to the second, organic; the latter 

 being common to animals and vegetables, and presiding over the organic 

 functions of nutrition, absorption, exhalation, secretion, &c. ; the former 

 existing only in animals, and presiding over the sensations, internal as 

 well as external. Animal sensibility will be considered here. It 

 would be well, indeed, to restrict the term sensibility to cases involving 

 consciousness. 



Pursuing the plan already laid down, the study of this interesting 

 and elevated function will be commenced, by pointing out, as far as 

 may be necessary, the apparatus that effects it, the nervous system. 



1. NERVOUS SYSTEM. 



Under the name nervous system, anatomists include all those organs 

 that are composed of nervous or pulpy tissue neurine. In man, it is 

 constituted of three portions: first, of what has been called the cerebro- 

 spinal axis, a central part having the form of a long cord, expanded at 

 its superior extremity, and contained within the cavities of the cranium 

 and spine ; secondly, of cords, called nerves, in number thirty-nine pairs, 

 according to some, forty-two, according to others, passing laterally 

 between the cerebro-spinal axis and every part of the body; and, lastly, 

 of a nervous cord, situate on each side of the spine, from the head to 

 the pelvis, forming ganglia opposite each vertebral foramen, and called 

 the great sympathetic. 



