98 TEXT-BOOK OF PHYSIOLOGY. 



The afferent nerves may also be classified, in accordance with their 

 distribution and the character of the sensations or other modes of nerve 

 activity to which they give rise, into several groups, as follows: 



1. Tegumentary nerves, comprising those distributed to skin, mucous mem- 



branes and sense organs and which transmit nerve impulses from the 

 periphery to the nerve centers. They may be divided into reflex and 

 sensorifacient nerves. 



A. Reflex nerves, those which transmit nerve impulses to the spinal 

 cord and medulla oblongata, where they give rise to different 

 modes of nerve activity. They may be divided into: 



1. Reflex excitator nerves, which transmit nerve impulses which 

 cause an excitation of nerve centers and in consequence in- 

 creased activity of peripheral organs, e.g., skeletal muscles, 

 glands, blood-vessels and viscera. 



2. Reflex inhibitor nerves, which transmit nerve impulses which 

 cause an inhibition of nerve centers and in consequence, 

 decreased activity of the peripheral organs. It is quite prob- 

 able that one and the same nerve may subserve both sensation 

 and reflex action, owing to the collateral branches which are 

 given off from the afferent roots as they ascend the posterior 

 column of the cord. 



B. Sensorifacient nerves, those which transmit nerve impulses to the 

 brain where they give rise to conscious sensations. They may be 

 subdivided into: 



1. Nerves of special sense e.g., olfactory, optic, auditory, 

 gustatory, tactile, thermal, pain, pressure which give rise to 

 correspondingly named sensations. 



2. Nerves of general sense e.g., the visceral afferent nerves 

 those which give rise normally to vague and scarcely perceptible 

 sensations, such as the general sensations of well-being or dis- 

 comfort, hunger, thirst, fatigue, sex, want of air, etc. 



2. Muscle nerves, comprising those distributed to muscles and tendons and 



which transmit nerve impulses from muscles and tendons to the brain 

 where they give rise to the so-called muscle sensations, e.g., the direction 

 and the duration of a movement, the resistance offered and the posture 

 of the body or of its individual parts. 



PHYSIOLOGIC PROPERTIES OF NERVES. 



Nerve Irritability or Excitability and Conductivity. These terms 

 are employed to express that condition of a nerve which enables it to develop 

 and to conduct nerve impulses from the center to the periphery, or from 

 the periphery to the center, in response to the action of stimuli. A nerve 

 is said to be excitable or irritable so long as it possesses these capabilities or 

 properties. For the manifestation of these properties the nerve must 

 retain a state of physical and chemic integrity; it must undergo no change 

 in structure or chemic composition. The irritability of an efferent nerve is 

 demonstrated by the contraction of a muscle, by the secretion of a gland, or 

 by a change in the caliber of a blood-vessel, whenever a corresponding nerve 

 is stimulated. The irritability of an afferent nerve is demonstrated by the 



