THE SPINAL CORD AND BRAIN 



815 



AXONE SHOWING 



FIERI LLAR 

 STRUCTURE 



INCISURE OF 

 SCHMIDT 



-NEURILEM MA 



branches soon after its departure from the cell; such axones are called dendraxones. The 

 axones and their collaterals end in terminal arborizations, the telodendria. 



The axone is the distributive or emissive (celli/ifugal) conductor of nerve impulses. There 

 is, therefore, a functional opposition attributable to the two extremities of the neurone, based 

 upon its dynamic polarity and upon a physiologic principle which is established by all experi- 

 ments to which the nerve system is submitted, 

 namely, that nerve impulses pass through the neu- 

 rone in a definite direction which is invariable and 

 admitting of anatomic localization. 



The majority of the peripheral spinal and cerebral 

 axones as well as those constituting the white sub- 

 stance of the brain and cord are invested by a myelin 

 sheath. 



The Collaterals (paraxones). The collaterals are 

 accessory branchings of the axones which are more 

 numerous in the cytoproximal portion and are usually 

 directed at right angles to the parent stem. Some 

 axones possess few or no collaterals, while others 

 possess many. The collaterals, especially those in the 

 gray substance of the central axis, are frequently mye- 

 linic. They unquestionably play an important part 

 in the grouping and chaining of neurones within the 

 system, in yielding up to neighboring neurones a por- 

 tion of the impulse that the cell has received by its 

 dendrites and transmits along its axone to a distance. 



Varieties Of Axones. Axones are divided into 

 two main groups depending upon the presence or 

 absence of a myelin sheath myelinic axones and 

 amyelinic axones, or medullated and nonmedullated 

 axones. 



Myelinic axones or medullated axis-cylinder 

 processes are axones enveloped by a relatively thick 

 sheath composed of semifluid phosphorized fat, which 

 gives to the bundles of these structures their opaque, 

 white appearance. The myelin sheath is in turn 

 invested by a delicate membrane (neurilemma) in 

 one group, while another group is devoid of such 

 covering, giving rise to the further subdivision into 

 (a) myelinic axones with a neurilemma; (fe) myelinic 

 axones without a neurilemma. 



Myelinic axones with a neurilemma constitute 

 the bulk of the cerebrospinal nerves, and, in lesser 

 proportion, of the sympathetic nerves. The myelin 

 sheath (medullary sheath of Schwann) (Figs. 587 and 

 oSS) does not invest the axone throughout its course nor 

 in a uniform manner. The axone after its emergence 

 from the cell body and likewise in its preterminal por- 

 tion is naked: and the delicate external membrane 

 or neurilemma comes in contact with the axone. 

 The myelin sheath consists of a number of tubular 

 segments demarcated by nodal intersections which 

 are only 0.08 mm. apart in the very small myelinic 

 axones, while for large axones the intervals may be 

 1 mm. or more. At the nodes (constrictions of Ran- 

 vicr) the neurilemma dips into the constriction to 



come in contact with the axone^ajid anv branches of th 



.such points. The interruptions in the continuity of the myelin sheath have been assumed to 

 be provisions facilitating nutritive diffusion between the axone and the surrounding lymph, 

 and here only may collaterals be given off. Each internodal myelinic segment is further char- 

 acterized by oblique clefts, irregularly distributed the incisures of Schmidt-Lantermann 

 seen only in fixed specimens and probably artefacts. Extraction of the fatty substance of 

 the myelin sheath by boiling alcohol and ether brings out a fine network which resists trypsin 

 digestion, and is termed neurokeratin on account of its resemblance to the keratin of epidermal 

 structures. 



The neurilemma (primitive sheath of Schwann; neurolemma), a delicate structureless mem- 

 brane, encloses the mvelin and the axone. wherever the myelin sheath is wanting. Against the 



FIG. 588. A. Amyelinic axones with a 

 neurilemma only, the nuclei of which can be 

 seen. B. Diagram showing structure of a 

 myelinic axone and illustrating two views 

 regarding the relations of the sheaths at the 

 node (compare the two sides). C. Trans- 

 section of a group of myelinic axones, stained 

 with psmic acid, showing: ^F. Axonic neu- 

 rofibrils. M. Myelin. /'. Kndoneurium. 



e axone are invariably piven o 



