6G GENERAL ANATOMY. 



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 plex- 

 uses, and occasionally by the terminal fasciculi, 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 indi- 

 vidual fasciculi become interlaced most intricately ; so that each branch leav- 

 ing a plexus may contain filaments from each of the primary nervous trunks 

 which form it. In the formation also of the smaller plexuses at the periphery 

 of the body, there is a free interchange of the fasciculi and primitive fibrils. 

 In each case, however, the individual filaments remain separate and distinct, 

 and do not inosculate with each other. 



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 nerve consists of tubular and gelatinous fibres, intermixed 

 with a varying proportion of filamentous areolar tissue, and inclosed in a 

 sheath formed of fibro-areolar tissue. The tubular fibres are, for the most 

 part, smaller than those composing the cerebro-spinal nerves; their double 

 contour is less distinct, and, according to Eemak, they present nuclei similar 

 to those found in the gelatinous nerve-fibres. Those branches of the sym- 

 pathetic which present a well-marked gray color, are composed more especially 

 of gelatinous nerve-fibres, intermixed with a few tubular fibres; whilst those 

 of a white color contain more of the tubular fibres, and few gelatinous. 

 Occasionally the gray and white cords run together in a single nerve, without 

 any intermixture, as in the branches of communication between the sym- 

 pathetic ganglia and the spinal nerves, or in the communicating cords between 

 the 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 

 afferent nerves, 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 impres- 

 sions either exciting muscular contractions, or influencing the processes of 

 nutrition, growth, and secretion. 



Terminations of Nerves. By the expression "the termination of nerve-fibres" 

 is signified their connections with the nerve centres, and with the parts which 

 they supply. The former are called their central, the latter their peripheral 

 terminations. With regard to the central terminations of the nerves, little is 

 as yet certainly known.' The nerve-cells, or nerve-corpuscles, above figured, 

 have been regarded as the central origin of the fibres with which they are 

 connected ; and it is very probable that in many cases they are so. There are 

 instances, however, in which such cells occur as mere nucleated swellings in 

 the course of a nerve, and in these cases they obviously cannot be regarded as 

 being in any sense the origins of the nerves. In other cases, as in the nerve- 

 cells in the anterior horn of the gray matter of the cord, there are numerous 

 processes springing out of the cell ; one of thesB (and according to Deiters one 

 only) is .recognized as an axis-cylinder ; the others are fibrilla3, which are con- 



1 The most recent author, and one of the most distinguished observers on this subject, Max 

 Schultze, speaks thus : " In the present state of our knowledge, we are not in a position to 

 assign its central origin to any single primitive fibril of the nervous system, however certainly 

 we may have discovered the peripheral terminations of a great part of them." (Schultze. in 

 Strieker's Handbuch, 1868, p. 134.) 



