254 A Manual of Veterinary Physiology. 



found. It is through the medium of this system that the 

 blood supply to a part is governed, that the tissues are 

 nourished, that the part possesses sensation and power of 

 movement, and that those processes of life are kept up over 

 which we have no control, and of the existence of which, 

 so far as our feelings are concerned, we have but little 

 knowledge ; further, it endows the body with sight, touch, 

 smell, and hearing, and furnishes animals with whatever 

 intelligence they are capable of exhibiting. The nervous 

 system of man and the lower animals principally differs on 

 the ground of intelligence ; it is this latter which produces 

 in man his great cerebral development, and marks him out 

 as a superior being. 



The nervous system is composed of the brain and spinal 

 cord, the nerves with their terminal endings, also a special 

 system of nerves known as the sympathetic. It is neces- 

 sary to the elucidation of the subject that these should be 

 dealt with in an inverse order, and we have first to inquire 

 into the function and properties of the nerves. 



The Nerves. 



These form the lines of communication between the 

 tissues and the brain and spinal cord. It is by means 

 of them that sensations arise, that motion is produced, that 

 the calibre of the bloodvessels is regulated, that secretion is 

 brought about, that the contraction of the heart is induced 

 and kept under control, that nutrition is governed, and 

 sight and other special senses produced. Nerves are, 

 therefore, spoken of as sensory, motor, vascular, secretory, 

 inhibitory, trophic, and nerves of special sense. These are 

 further subdivided into those whose duty is to convey im- 

 pulses from the body to the brain and spinal cord, and 

 hence called centripetal or afferent nerves, whilst those 

 which transmit impulses from the brain and spinal cord 

 to the body are termed centrifugal or efferent nerves. 



We have no means of distinguishing these various nerves 

 either by their physical properties or microscopicaJ appear- 

 ance; and though in the body their activity is exercised 



