544 TEXT-BOOK OF PHYSIOLOGY 



cerebellar tract. By this means the cord and cerebellum are associated ir 

 functional activity. 



Spino-encephalic or Sensor Conduction. The spino-encephalic 01 

 sensor pathway extends from different levels of the spinal cord to the cortex 

 of the cerebrum, and, consists of two or more consecutively arranged groups 

 of neurons. The first group of these neurons extends from the spinal cord 

 to the thalamus and is known as the spino-thalamic system; the second group 

 of these neurons extends from the thalamus to the cortex of the cerebrum and 

 is known as the thalamo-cortical system. 



The spino-thalamic system is connected with the cutaneous and mucous 

 surfaces, and with the tendons, joints and muscles by means of the dorsa' 

 root fibers of the spinal nerves. Histologic and clinical investigations have 

 shown that the central terminations of the nerve-fibers corning from the skin 

 become associated with afferent cells on the same side of the gray matter 

 at the same or at somewhat different levels; that the axons of these cells 

 cross the median plane and then pass upward through the cord, medulla 

 pons and crura cerebri to the thalamus. Similar methods of invest] gati or 

 have shown that the nerve-fibers coming from the tendons, joints, and mus- 

 cles ascend the white matter of the spinal cord of the same side, as far as the 

 upper limits of the posterior column, where their terminal branches become 

 associated with the cells of the cuneate and gracile nuclei. From the cells 

 of these nuclei, axons arise which cross the middle plane of the medulla tc 

 join the fibers ascending the spinal cord. The former fibers are known as 

 the internal arcuate fibers and assist in the formation of the lemniscus o] 

 fillet (Fig. 233). The afferent or sensor pathway thus decussates in part, 

 at different levels of the spinal cord, and in part at the level of the gracik 

 and cuneate nuclei. The former is often termed the lower, the latter the 

 upper sensor decussation or crossway. 



The afferent pathway on passing toward the thalamus receives additional 

 fibers at the level of the medulla and pons from the cells of the opposite 

 side with which the terminations of the afferent cranial nerves, the trigem- 

 inal, glossopharyngeal, and vagus, are associated. 



The nerve impulses that arise in consequence of impressions made on 

 the terminals of the nerves in the cutaneous and mucous surfaces, in the 

 viscera and in the muscles, are transmitted through the dorsal roots of the 

 spinal nerves to the cord. On reaching the cord the cutaneous nerve im- 

 pulses are received by afferent or receptive nerve-cells in the gray matter, 

 and transmitted by their axons across the median plane of the spinal cord 

 and then upward to and through the medulla, the posterior part of the pons, 

 the posterior part of the crura cerebri, and for the most part to the ventral 

 portion of the thalamus opticus. 



On reaching the thalamus the nerve impulses are received by nerve-cells, 

 and then transmitted by their axons which pass by way of the internal cap- 

 sule to the cells of the cortex of the cerebrum. It is probable however that 

 some fibers from the cord and medulla pass direct to the cortex. When 

 thus transmitted ^through the cord to the cerebral hemispheres directly or 

 indirectly, they are received by specialized nerve-cells in the cortex and trans- 

 lated into conscious sensations. The nerve impulses developed in tendons 

 and muscles are received by localized groups of nerve-cells, the nucleus 

 gracilis and nucleus cuneatus, situated at the top of the posterior column, 



