102 DISEASES OF CATTLE. 



instantly conveyed to the brain. The terminal end organs of the 

 nerves that supply the muscles are different, as they give the impulse 

 which is conveyed by the motor nerves to the elements which consti- 

 tute the muscle, and this impulse is the excitation which causes the 

 muscle to contract. The terminal end organs of the special senses 

 of taste, smell, etc., receive their special impressions, and their respec- 

 tive nerves carry the impressions to the brain. 



There are two divisions of nerves, the afferent and efferent. 



The afferent nerves are those which convey the impression to the 

 nerve centers. All the sensory nerves belong to this division. 



The efferent nerves are those which convey the nervous impulse 

 outward from the nerve centers, and they are further classified ac- 

 cording to the function of their respective centers. For example: 

 Motor fibers carry the impulse from the nerve center to a muscle to 

 cause contraction. Vasomotor fibers carry the impulse to the muscu- 

 lar tissue in the blood vessels, which regulates their caliber. The 

 secretory fibers convey the impulse to the cells of the glands and 

 excite the activity of the ghind, and its particular product is secreted 

 or evolved, as, for instance, milk in the mammary gland. Inhibitory 

 fibers control or inhibit the action of the organ to which they are 

 distributed, as, for instance, the heart. 



Nerve centers may be considered as a collection or gi-oup of nerve 

 cells. Both the cerebrospinal and the sympathetic divisions have 

 nerve centers. The centers derive their special names from their 

 functions. The brain is the great center of the nervous system, as it 

 is the center of intelligence and perception. The centers of all the 

 special senses, as well as the centers of various functions, are located 

 in different parts of the brain. Nerve centers also exist in the spinal 

 cord and in connection with the sympathetic system. 



A nerve is a cord consisting of a certain number of fibers of nerve 

 tissue, inclosed in a sheath of connective tissue. Nerves divide and 

 subdivide, sending off branches^ which ramify in all parts of the 

 body, and, as they near their terminations, they contain but one or 

 two fibers. 



The hrain and spinal cord are contained within a bony canal, which 

 forms a protective covering for them. 



The spinal cord, or spinal marrow, lodged within the spinal canal, 

 or hollow of the backbone, is continuous with the brain anteriorly, 

 and terminates in a point in the sacrum (that part of the spinal 

 column which immediately preceds the tail). The spinal cord gives 

 off branches at each of the spaces between the segments of the back- 

 bone. These branches form nerve trunks which carry both sensory 

 and motor impressions and impulses. The spinal cord is a grand 

 nerve trunk to carry m?ssages to or from the brain and to and from 

 the reflex centers contained within itself. 



