GENERAL PHYSIOLOGY OF NERVE-TISSUE 93 



the brain and spinal cord the tufts are in more or less intimate relation 

 with the dendrites of adjacent neurons. 



The neurons in their totality constitute the neuron or nerve tissue. 

 From the fact that they are arranged both serially and collaterally into a 

 regular and connected whole, they collectively constitute a system known as 

 the neuron or nerve system. 



The neurons composing the spinal and cranial nerves are represented in 

 Fig. 44, which are connected peripherally by their terminal branches with 

 muscles on the one hand and with epithelium of skin, mucous membrane, etc., 

 on the other hand. In the spinal cord the terminal branches of the afferent 

 neuron come into histologic and physiologic relation with the dendrites of a 

 second neuron, the axonic process of which in many instances ascends the 

 cord to different levels or even as far as the brain, where its terminal branches 

 come into relation with the dendrites of still another neuron, the axonic 

 process of which is in turn connected with neurons in the cortex of either 

 the cerebrum or cerebellum. The surfaces of the body are thus brought into 

 relation with the cerebral and cerebellar neurons. The neurons arranged 

 in this serial manner constitute the afferent side of the nerve system. 



In a similar way the efferent neurons of the spinal and cranial nerves are 

 brought into relation with the cortex of the cerebrum. Large pyramidal- 

 shaped neurocytes situated in specialized regions of the cortex of the cere- 

 brum send their axonic processes down through the brain and cord. As they 

 approach their destination the terminal branches become related histo- 

 logically and physiologically with the dendrites of the neurons composing 

 the cranial and spinal nerves. The cortex of the cerebrum is thus brought 

 into relation with the general musculature of the body. The neurons 

 arranged in this serial manner constitute the efferent side of the nerve system. 



Neurons, moreover, are grouped into more or less complexly organized 

 masses, termed organs, which in accordance with their locations may be 

 divided for convenience into central and peripheral organs. 



The Central Organs of the Nerve System. The central organs con- 

 sist of the encephalon and spinal cord, contained within the cavities of the 

 head and spinal column respectively. They consist of neurons arranged 

 in a very complex manner. In a subsequent chapter the anatomic arrange- 

 ment of their constituent parts will be detailed. 



The Peripheral Organs of the Nerve System. These consist of the 

 cranial and spinal nerves and the sympathetic ganglia. Each nerve 

 consists of a variable number of nerve-fibers united into firm bundles by 

 connective tissue which supports blood-vessels and lymphatics. The bundles 

 are technically known as nerve-trunks or nerves. 



The nerve-trunks connect the brain and cord with all the remaining 

 structures of the body. Each nerve is invested by a thick layer of lamel- 

 lated connective tissue, known as the epineurium. A transverse section of a 

 nerve shows (see Fig. 44), that it is made up of a number of small bundles of 

 fibers, each of which possesses a separate investment of connective tissue 

 the perineurium. Within this membrane the nerve-fibers are supported by 

 a fine stroma the endoneurium. After pursuing a longer or shorter course, 

 the nerve-trunk gives off branches, which interlace very freely with neigh- 

 boring branches, forming plexuses, the fibers of which are distributed to 

 associated organs and regions of the body. From their origin to their 



