100 DISEASES OF CATTLE. 



conveyed by the motor nerves to the elements which constitute 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 respective 

 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. Ail the sensory nerves belong to this division. 



The efferent nerves are those which convey the nervous impulse out- 

 ward from the nerve centers, and they are further classified according 

 to the function of their respective centers. For example : Motor fibers 

 carry the impulse from the nerve center to a muscle to cause contrac- 

 tion. Vaso-motor fibers carry the impulse to the muscular 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 gland, 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 group of nerve 

 cells. Both the cerebro-spinal 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 brain 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 precedes 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 messages to or from the brain and to and from 

 the reflex centers contained within itself. 



The brain is contained within the cavity of the skull and is con- 

 tinuous with the spinal cord; there is nothing to mark the place 

 where one leaves off and the other begins. The brain is the seat of 



