INHIBITORY AND AFFERENT NERVES. 583 



[There are other forms due to nervous disease, including symmetrical gangrene and local 

 asphyxia of the terminal parts of the body, such as toes, nose, and external ear, caused perhaps 

 by spasm of the small arterioles (Raynaud's disease) ; and the still more curious condition of 

 perforating ulcer of the foot. Haemorrhage of nervous origin sometimes occurs in the skin, 

 including those that occur in locomotor ataxia after severe attacks of pain, and haematoma 

 aurium, or the insane ear, which is specially common in general paralytics.] 



(d) [Inhibitory nerves are those nerves which modify, inhibit, or suppress a 

 motor or secretory act already in progress.] 



Take as an example the effect of the vagus upon the action of the heart. Stimulation of the 

 peripheral end of the vagus causes the heart to stand still in diastole ( 85) ; see also the 

 effect of the splanchnic upon the intestinal movements ( 161). The vaso-dilator nerves, or those 

 whose stimulation's followed by dilatation of the blood-vessels of the area which they supply, 

 are referred to especially in 237. 



[There is the greatest uncertainty as to the nature and mode of action of inhibitory nerves, 

 but take as a type the vagus, which depresses the function of the heart, as shown by the slower 

 rhythm, diminution of the contractions, relaxation of the muscular tissue, lowering of the 

 excitability and conduction. These phenomena are not due to exhaustion. Gaskell points out 

 that the action is beneficial in its after effects, so that this nerve, although it causes 

 diminished acitivity, is followed by repairjof function ; hence, he groups it as anabolic nerve, 

 the outward symptoms of cessation of function indicating that constructive chemical changes 

 are going on in the tissue.] 



(e) Thermic and electrical nerves have also been surmised to exist. 



[Gaskell classifies the efferent nerves differently. Besides motor nerves to striped muscle, 

 he groups them as follows : 



1. Nerves to vascular muscles. 



(a) Vaso-motor, i.e., vaso-con stricter, accelerators and augmentors of the heart. 



(b) Vaso-inhibitory, i.e., vaso-dilators and inhibitors of the heart. 



2. Nerves of the visceral muscles. 



(a) Viscero-motor. 



(b) Viscero -inhibitory. 



3. Glandular nerves.] 



[Other terms are applied to nerves with reference to the chemical changes 

 they excite in a tissue in which they terminate. The ordinary metabolism is the 

 resultant of two processes one constructive, the other destructive, or of assimila- 

 tion and dissimilation respectively. The former process is anabolism, the latter 

 katabolism. A motor nerve excites chemical destructive changes in a muscle, and 

 is so far the katabolic nerve of that tissue; in the same way the sympathetic to 

 the heart, by causing more rapid contraction, is also a katabolic nerve, while the 

 vagus, as it arrests the heart's action, and brings about a constructive metabolism 

 of the cardiac tissue, is an anabolic nerve (Gaskell).] 



II. Centripetal or Afferent Nerves. (a) Sensory Nerves (sensory in the 

 narrower sense), which by means of special end-organs 

 conduct sensory impulses to the central nervous system. 



(b) Nerves of Special Sense. 



(c) Reflex or Excito-motor Nerves. When the 

 periphery of one of these nerves is stimulated, an im- 

 pulse is set up which is conducted by them to a nerve- 

 centre, from whence it is transferred to a centrifugal or 

 efferent fibre, and the mechanism (I, a, b, c, d) in con- 

 nection with the peripheral end of this efferent fibre is 

 set in action ; thus, there are Reflex motor, Reflex 

 secretory, and Reflex inhibitory fibres. [Fig. 422 Fig. 422. 



shows the simplest mechanism necessary for a reflex Scheme of a reflex motor act. S, 



motor act. The impulse starts from the skin, S, travels skin > % aff f jt nerve ; N, 



^ j, * A , ' ' , T nerve-cell ; ef, efferent fibre. 



up the nerve, af, to the nerve-centre or nerve-cell, jn, 



situate in the spinal cord, where it is modified and transferred to the outgoing fibre, 



ef, and conveyed by it to the muscle, M.] 



III. Intercentral Nerves. These fibres serve to connect ganglionic centres 



