180 INTRODUCTION TO NEUROLOGY 



fibers (cf. Figs. 66, 67, pp. 150, 151) and they lack the medial lemniscus 

 altogether. The spino-cerebellar tracts, on the other hand, are very ancient 

 and are present from the lowest to the highest vertebrates. 



These considerations suggest that the first fibers to pass from the spinal 

 cord to the higher centers of the brain, and presumably the first sensory 

 impulses from the spinal nerves to be consciously perceived, were those of 

 touch and temperature transmitted through the spinal lemniscus. (Pain 

 is probably also very primitive as a conscious experience, but it is doubtful 

 whether it is represented in the spinal lemniscus of lower forms; see p. 251). 

 The proprioceptive impulses in lower vertebrates are coordinated quite un- 

 consciously in the brain stem and cerebellum, and it is only in the higher 

 forms that this system of nervous impulses reaches the thalamus (through 

 the medial lemniscus) and cerebral cortex for conscious control. Clinical 

 evidence shows that the medial lemniscus connections in man are concerned 

 with the conscious adjustments of the positions and orientation in space of 

 the body and its members and with spatial discriminations of various sorts, 

 rather than with the senses of touch and pressure as externally projected. 



The innervation of the organs of muscular sensibility and tendon sensi- 

 bility in the head is not as fully known as in the case of those of the trunk 

 and limbs, as above described. Sherrington and Tozer have recently 

 shown that such organs are present in the muscles which move the eyeball 

 and that their nerves accompany the motor fibers of the III, IV, and VI 

 cranial nerves; but of the central connections of these sensory nerve-fibers 

 of the eye muscles nothing is known. It is suggested by the researches of 

 Johnston, Willems, and many others that the jaw muscles, which receive 

 their motor innervation from the motor V nucleus (nucleus masticatorius), 

 receive their sensory innervation from the mesencephalic nucleus of the V 

 nerve, whose position along the lateral border of the aqueduct of Sylvius 

 is seen in Figs. 71, 75, and 77. But recent studies of Edgeworth have 

 shown that these muscles also receive sensory fibers from the semilunar 

 or Gasserian ganglion of the V nerve, and the question requires further inves- 

 tigation. Possibly the sensory fibers from the Gasserian ganglion to the 

 muscular branches of the V nerve conduct impressions of deep sensibility 

 of pressure and pain of the exteroceptive type, while those from the mesen- 

 cephalic V nucleus innervate the muscle spindles for true proprioceptive 

 sensibility. 



The fibers of the chief sensory root of the V nerve in part end in the chief 

 sensory V nucleus near the level of their entrance into the medulla oblongata 

 (Figs. 71, 77) and in part pass downward through the whole length of the 

 medulla oblongata and upper levels of the spinal cord as the spinal V tract 

 (Figs. 64, 71, 72, 81). It is suggested by clinical and comparative evidence 

 that the spinal V tract and its nucleus are connected with a phylogenetically 

 old type of reaction to touch, temperature, and pain, probably chiefly reflex, 

 while the chief nucleus is concerned with the more recently acquired dis- 

 criminations of these systems with more direct cortical connections. The 

 fibers of the trigeminal lemniscus (p. 157) follow two separate tracts arising 

 from these two parts of the sensory V nucleus, only the upper one of which 

 is shown in Fig. 77, though both are shown in Fig. 81 (neurons 8 and 9). 



Motor Paths. Throughout the length of the spinal cord and 

 brain stem the ascending fibers of both exteroceptive and pro- 

 prioceptive sensibility give off collateral branches into the reticu- 



