762 THE NERVOUS SYSTEM 



FUNDAMENTALS OF CONSTRUCTION 



The functionally mature nervous system consists of peculiarly differentiated 

 essential cell elements held in place by two forms of supporting tissue and supplied 

 with abundant blood-vessels. 



The nervous element is distinguished from all other units of the structure of 

 organs in that its cell-body gives off outgrowths or processes of peculiarly great 

 length and characteristic form. Knowledge of the possible lengths and com- 

 plexity of these processes is comparatively recent and, to include them together 

 with their parent cell-body, which has long been known as the nerve cell, the 

 term neurone is used. The neurone, therefore, may be defined as the nerve cell- 

 bod}' with all its processes, however numerous and far reaching they may be. As 

 a class of tissue elements, all neurones possess characteristics distinguishing 

 them from other tissue elements, but the varieties within this class vary greatly. 

 They vary in form both according to function and according to their locality in 

 the nervous system. They vary in different animals, those in the higher animals 

 being more complex in form. Fig. 604 gives illustrations of the external form of 

 the cell-body of a few of the types found in the human nervous system. 



The cell-body of the neurone gives off two general types of processes, 

 dendrites and axones: 



(1) The dendritic processes or dendrites. These are the more numerous, the shorter, and 

 the more frequently branching processes. They branch dichotomously and with rapid decrease 

 in diameter as they branch. They serve to increase the absorbing surface of the cell-body 

 for purposes of nutrition. Nerve impulses transmitted to the neurone are received by them and, 

 therefore, they also serve to increase the recipient surface of the neurone. They never acquire 

 medullary sheaths. Since they convey impulses toward the cell-body, they are known as 

 cellipital processes. Their absorbing and receptive surfaces are further increased by the presence 

 of thickly placed, very minute projections known as "pin-head processes" or gcmmules. 



(2) The axone (neuraxis). Each neurone possesses properly but one of these processes. 

 It arises from the cell-body more abruptly and quickly becomes smaller in diameter than are 

 most dendrites before the latter decrease by branching. It is the longest process, in most cases 

 very much longer than dendrites. Computation shows that some axones may contain nearly 

 200 times the volume of the parent cell-body of the neurone. Occasionally the axone gives off 

 a few small branches near the cell-body. These are known as collaterals and are given off at 

 practically right angles instead of dichotomously. Regardless of its branching, the axone 

 maintains a practically uniform diameter throughout its long course. Its usual nervous func- 

 tion is to convey the impulses away from the cell-body, either to transmit them to other neu- 

 rones by contact upon their dendrites, etc., or to appropriate elements of the other tissue 

 systems of the body. Thus the axones are the cellifugal processes. There is one well-known 

 partial exception to this, namely, a part of the axone of the spinal ganglion type of neurone, 

 the peripheral sensory neurone. The axone of this bifurcates a short distance from the cell- 

 body into a peripheral and a central branch. See fig. 604, A, and fig. 610. The peripheral 

 branch collects sensory impulses from the tissues of the body, the skin, etc., and, in conveying 

 them to the central system, must necessarily convey them toward the cell-body as far as the 

 point of bifurcation. Thence the impulse goes on in the central branch, still toward the central 

 system but now, in conformity, away from the cell-body of the neurone. While the continued 

 vitality of the axone is dependent upon the cell-body, in the peculiar case of the spinal ganglion 

 neurone the impulse does not necessarily pass through the cell-body. Experiments with the 

 lower animals have shown that the impulses pass in the fibre from the peripheral tissues to the 

 central system when the cell-body has been cut away. 



Terminations of axones. — At its final termination, well beyond its collateral branches and 

 usually a considerable; length from its cell-body, the axone practically always divides into two 

 or more terminal branches, and each of these breaks up, now dichotomously, into numerous 

 terminal twigs. These terminal twigs are known as telodendria. Telodendria vary in number 

 and character of form according to the tissues in and upon which they terminate. Functionally, 

 they are of three clas.ses: Those terminating upon and in the other (peripheral) tissues of the 

 body are either (1) sensor i/ or (2) motor. In order to transmit impulses from one neurone to 

 another, tclfxlcndria of the axone of one neurone are placed in contact with the dendrites or 

 cell-body of another neurone forming ('.\) synapses. Upon approaching its termination, every 

 axone loses its shciath, its telodendria being necessarily bare. 



Sensory or afferent axones, receiving iinpul.ses from the skin or other epithelial surfaces, 

 break up into very numerous fekxlendria each of which terminates directly upon the surface 

 of the epithelial cell, such as the cells of the gerniinative (Malpigin) layer of the skin or 

 tho.se of its basal or columnar layer. Such telodendria are known an free terminations. Free 

 terminations are also to be found in the connective tissues of the body. A second variety of 

 peri[)licral lermination of afferent axones is the enrapsiilated form. These are known as 'end 

 organs' and '(•ori)uscles' and are named according to their complexity and position. Three 

 of the different forms of them are shown in fig. OOf), B, C, and D. These are always situated 

 in fibrous connective tissue from wliich their capsules are derived. Their most elaborate form 

 is the lamellated or Pacinian cor|)us(;le. Besides the motor axones terminating upon the 

 fibres of voluntary or skeletal muscle, .sensory impulses are carried from this tissue and one of 

 the forms of telodendria for this purpose terminates upon the muscle fibre. This is known as 



