NERVOUS TISSUE. 75 



and at their junction with the branches of neighboring nerves the filaments pass 

 to become intermixed with those of the other nerves in their further progress ; in 

 no instance, however, have the separate nerve-fibres been shown to inosculate. 



The communications which take place between two or more nerves form what 

 is called a plexus. Sometimes a plexus is formed by the primary branches of the 

 trunks of the nerves as the cervical, brachial. lumbar, and sacral plexuses and 

 occasionally by the terminal funiculi, as in the plexuses formed at the periphery 

 of the body. In the formation of a plexus the component nerves divide, then 

 join, and again subdivide in such a complex manner that the individual funiculi 

 become interlaced most intricately ; so that each branch leaving a plexus may 

 contain filaments from each of the primary nervous trunks which form it. In the 

 formation also of smaller plexuses at the periphery of the body there is a free 

 interchange of the funiculi and primitive fibres. In each case, however, the 

 individual filaments remain separate and distinct, and do not inosculate with one 

 another. 



It is probable that through this interchange of fibres the different branches 

 passing off from a plexus have a more extensive connection with the spinal cord 

 than if they each had proceeded to be distributed without such connection with 

 other nerves. Consequently the parts supplied by these nerves have more extended 

 relations with the nervous centres : by this means, also, groups of muscles may be 

 associated for combined action. 



The sympathetic nerves are constructed in the same manner as the cerebro- 

 spinal nerves, but consist mainly of non-medullated fibres, collected into funiculi. 

 and enclosed in a sheath of connective tissue. There is, however, in these nerves 

 a certain admixture of medullated fibres, and the amount varies in different 

 nerves. Those branches of the sympathetic which present a well-marked gray 

 color are composed more especially of non-medullated nerve-fibres, intermixed 

 with a few medullated fibres ; whilst those of a white color contain more of the 

 latter fibres and a few of the former. Occasionally, the gray and white cords 

 run together in a single nerve, without any intermixture, as in the branches of 

 communication between the sympathetic ganglia and the spinal nerves, or in the 

 communicating cords between the ganglia. These medullated fibres are derived 

 from the central nervous system through the rami communicantes, which pass from 

 the cerebro-spinal nerves to the various sympathetic ganglia. 



The nerve-fibres, both of the cerebro-spinal and sympathetic system, convey 

 impressions of a twofold kind. The sensory nerves, called also centripetal or 

 >i.rf'> / -tit nerve.-, transmit to the nervous centres impressions made upon the 

 peripheral extremities of the nerves, and in this way the mind, through the 

 medium of the brain, becomes conscious of external objects. The motor nerves, 

 called also centrifugal or efferent nerves, transmit impressions from the nervous 

 centres to the parts to which the nerves are distributed, these impressions either 

 exciting muscular contraction, or influencing the processes of nutrition, growth, 

 and secretion. 



Origin and Termination of Nerves. By the expression " the termination of 

 nerve-fibres " is signified their connection with the nerve-centres, and with the 

 parts they supply. The former are sometimes called their origin, or central 

 termination ; the latter their peripheral termination. The origin in some cases is 

 single that is to say. the whole nerve emerges from the nervous centre by a single 

 root ; in other instances the nerve arises by two or more roots, which come off 

 from different parts of the nerve-centre, sometimes widely apart from each other, 

 and it often happens, when a nerve arises in this way by two roots, that the 

 functions of these two roots are different ; as, for example, in the spinal nerves, 

 each of which arises by two roots, the anterior of which is motor and the posterior 

 sensory. The point where the nerve root or roots emerge from the nervous centre 

 is named the fupcrjicial or apparent origin, but the fibres of Avhich the nerve 

 consists can be traced for a certain distance into the nervous centre to some por- 

 tion of the gray substance, which constitutes the deep or real origin of the nerve. 



