94 HUMAN PHYSIOLOGY. 



viscera. They consist of a -dense, investing, fibrous membrane, containing 

 in its interior gray or vesicular cells, among which are found white and gela- 

 tinous nerve fibres. They may be regarded as independent nerve centres. 



Structure of Nerves. Within the cranial and spinal cavities, the nerve 

 fibres are bound together by connective tissue in the form of continuous 

 bundles. Through the foramina of these cavities the nerve fibres emerge 

 in the form of rounded or flattened cords which are termed nerves. Each 

 nerve is surrounded by a sheath of connective tissue, the neurilemma, 

 which also forms a stroma in which the blood vessels ramify, furnishing 

 nutritive material for the growth and repair of the ultimate nerve fibres. 



A Nerve consists of a greater or less number of ultimate nerve filaments, 

 separated into bundles by fibrous septa given off from the neurilemma. The 

 nerve filaments pursue an uninterrupted course, from their origin to their 

 termination; branches pass from one nerve trunk into the sheath of 

 another, but there is no anastomosis or coalescence with adjoining nerve 

 fibres. Nerves are channels of connection between the brain and cord, 

 and the muscles, glands, skin, mucous membranes, etc., in which they ter- 

 minate. Any excitation at either end produces in the nerve an impulse 

 which travels throughout the length of the fibre. If the nerve fibres going 

 to a muscle or gland are stimulated, there is increased muscular movement, 

 and increased secretion ; if the nerve fibres distributed to the skin or mucous 

 membranes are stimulated, there is produced in the brain a sensation. This 

 difference in effects produced by irritation has led to a division of fibres 

 into two classes: viz., I. Efferent or motor. 2. Afferent or sensory. There 

 is no anatomical or chemical difference discoverable between these two 

 classes of fibres. 



A Plexus is formed by a number of branches of different nerves inter- 

 lacing in every direction, in the most intricate manner, but from which 

 fibres are again given off to pursue their independent course, e. g., brachial, 

 cervical, lumbar, sacral, cardiac plexuses, etc. 



Nerve Terminations, (i) Central. Both motor and sensory nerve 

 fibres, as they enter the spinal cord and brain, lose their external invest- 

 ments, and retaining only the axis cylinder, ultimately become connected 

 with the processes of the gray cells. 



(2) Peripheral. As the nerves approach the tissues to which they are 

 to be distributed, they inosculate freely, forming a plexus from which the 

 ultimate fibres proceed to individual tissues. 



Motor Nerves. In the voluntary or striped muscles the motor nerves 

 are connected with the contractile substance by means of the " motorial 



