372 ANATOMICAL TECHNOLOGY. 



tion in the arteries and the motor nerves is centrifuga', while it is centripetal in the sensory 

 nerves and the veins. [For this reason, upon colored diagrams, it is at least convenient to 

 represent the motor nerves by red, the color of most arterial blood, and the sensory nerves 

 by blue, the color of the blood in most of the veins]. 



(E) Like the two kinds of vessels, the motor nerves divide and subdivide so as to form 

 smaller and smaller branches, while the sensory nerves unite and reunite to form larger 

 and larger trunks. 



1005. Differences between the Nervous and Vascular Systems. Here, however, the 

 strict analogy ceases. The connection of the two sets of nerves at the myelon is not well 

 understood, but it is certainly less simple than that of the great venous and arterial trunks 

 at the heart, separated as they are only by the lungs. 



Again, while the capillaries establish a complete continuity of the peripheral ends of 

 the arteries and veins, no such constant connection has been shown to exist between the 

 motor and sensory nerves, which commonly terminate independently either in muscle or 

 at sensitive surfaces ; but see Beale, A, 240. 



The subdivision of the vessels is like that of a large stream into several smaller, all 

 being part of one. But as a rule the nerve fibers maintain their independence through- 

 out their course from the myelon to the termination, and the larger nerves are made up 

 by the association of many fibers, and not by their actual union into one large fiber. 



Arteries and veins differ not only in that the former usually contain purer blood and the 

 latter that which is less pure (the pulmonary vessels forming exceptions to the rale) ; nor 

 even in that the current in the one is always toward the heart and in the other away 

 from it. The real distinction is in their structure, the arteries containing more elastic and 

 muscular tissue, by virtue of which they are more perfectly elastic and contractile ; their 

 walls also are relatively thicker, so that they remain open when empty instead of collaps- 

 ing like the veins. But no such distinctions have been ascertained between the motor and 

 sensory nerves, and the proof that a sensory nerve may transmit an impression in the 

 direction opposite to the usual one goes far to indicate that their properties as well as their 

 structure are identical, and that the difference in their functions depends upon the connec- 

 tions of their central end with the dorsal and ventral regions of the myelon and of their 

 peripheral ends with muscles or with sensitive parts. 



THE' MYELENCEPHALIC OR CEREBRO-SPINAL 

 NERVOUS SYSTEM. 



1006. The Myelon. This, called also chorda spinalis or 

 spinal cord, is the longer and more slender portion of the myelen- 

 cephalon or central part of the cerebro-spinal axis, and is lodged 

 within the canalis neuralis of the columna vertebralis ( 479). 



References to the Myelon. Quain, A, I, 568, and II, 489 ; Gray, A. 602 ; Hyrtl, A, 463 ; 

 Gegenbaur (Lankester), A, 512 ; Chauveau, A, 709 ; Chauveau (Fleming). A, 666 and 747 ; 

 Gurlt, A, 715 ; Owen, A, III, 73 ; Milne-Edwards, A, XI, 257 ; Leyh, A, 504. 



As briefly stated in 997, 998, the myelon is a continuous mass of alba and cinerea, 

 and is functionally a gigantic motor, and sensory nerve, an elongated ganglion and a cen- 

 ter of reflex actions. It may be conveniently divided into regions corresponding with 

 those of the vertebral column, cervical, thoracic, lumbar, sacral and caudal; but the ana- 

 tomical distinctions between them consist chiefly in the increase or decrease of the cinerea 



