96 PHYSIOLOGY. 



ANIMAL FUNCTIONS, 



OR FUNCTIONS OF RELATION. 



The animal functions are so called because they are peculiar to 

 that class of organized beings. They render the individual conscious 

 of external impressions, and capable of executing spontaneous move- 

 ments, and are dependent for their exercise on the existence of a 

 nervous system. They are thus classified : 



1st. Sensation. 



2d. Muscular Motion. 



3d. Mental Manifestation. 



It has been shown (pages 38, 39) that into the composition of the 

 nervous structure two distinct kinds of matter enter, viz. : ihe gray ov 

 vesicular, and the white, or tubular ; that these two kinds differ not 

 only in structure and colour, but also \n function, the gray, or vesi- 

 cular, being a generator of nervous influence, and the white the 

 carrier of this influence to the various parts of the body. A union 

 of these two kinds of matter constitutes a ganglion, or nervous 

 centre. 



Our fundamental idea of a nervous system consists of a ganglion, 

 or centre, thus composed, and a set of trunks composed of the whit^ 

 or tubular matter, connecting the central organ with the different 

 parts of the fabric. These trunks or branches are distributed to the 

 sensory surfaces or organs, and to the muscles or motor organs. The 

 first receive and convey impressions//-o;?x the periphery to the centre, 

 and are hence called afferent. The latter convey motor influence 

 from the centre to the periphery, and are hence called efferent. The 

 first are sometimes called sensory, from their connexion with sensa- 

 tion ; the latter are likewise sometimes designated motory, from their 

 connexion with muscular contraction. 



The functional activity of the nervous system is mainly depen- 

 dent upon the due supply of oxygenated blood ; this is especially 

 necessary at the points at which changes originate, not seeming so 

 necessary for the mere conduction of impressions. In accordance 

 with this, we find the centres, and the peripheral extremities of affe- 

 rent nerves always duly supplied with arterial blood ; any interrup- 

 tion to its supply being attended with an immediate arrest of their 



