ANATOMY IN A NUTSHELL. 433 



tions from the skin, mucous membranes, bones, special sense organs, to the 

 cortex, where are produced in an unknown way what we call sensation. Each 

 half of the body is thus represented in or projected upon the cortex of the opposite 

 hemisphere; hence the name of projection fibers given to those fibers of the 

 brain through which communication is maintained. (Plate CLXXXI). Thev 

 are in many instances collected into distinct bundles; so that we may speak of 

 motor and sensory tracts or paths, meaning thereby definite groups of fibers 

 along which motor or sensory impulses, as the case may be, travel. Moreover 

 these groups of fibers often arise from or terminate hi fairly definite areas of the 

 cortex, which areas are usually called centers. (Plate CLXXXI 1 1). Thus, 

 that from which springs the fibers that conduct impulses to the muscles which 

 produce the movements of the hand may be called the motor center of the hand; 

 that which receives the fibers along which visual impressions travel is the 

 center of sight. 



Of the many ways in which impulses may travel between the brain and the 

 spinal cord, the following (1), (2), (3), (4), (5), (6), are the most important that 

 have been demonstrated. If these impulses travel through the cerebellum we 

 call the way they pass the indirect route, but if they miss the cerebellum we call 

 the way they travel the direct route. This applies to both motor and sensory 

 impulses. 



Sensory impulses, which are afferent, (centripetal) take the following routes: 



(1.) They travel by the way of the tracts of Goll and Burdach 

 (fasciculus gracilis et cuteatus) (Plate CCXXX) to the nuclei gracilis et cuneatus 

 by means of neurones of the first order. By way of explanation we might say 

 that a neurone system consists of a group of nerve fibers and their cells of origin. 

 From nuclei gracilis et cunetus the impulses travel by neurones of the second 

 order through the mesial fillet to the optic thalamus. The mesial fillet crosses 

 to the opposite side and gives off collaterals to (a) the bulbar nuclei, (1)) to the 

 mid-brain. From the optic thalamus the impulses travel to the cerebral cortex 

 by the way of neurones of the third order, however some of those of the second 

 order may pass through the optic thalamus to the cerebral cortex uninterrupted. 

 Thus we see two neurone systems may carry the impulses to the cortex or it 

 may take three. Muscle and joint sensations travel through these i racts. This 

 route is direct because it does not travel through the cerebellum. 



(2.) After passing through neurones of the first order by the way of col- 

 laterals to the gray matter of the anterior horn, the impulses travel through the 

 AlNTERO-lateral- ground bundle (fasciculi proprii) by neurones oi the second 

 order to the optic thalamus. While in the posterior longitudinal bundle (fas- 

 ciculus longitudinalis medialis) collaterals are given to the bulbar nuclei and 

 mid-brain. From the optic thalamus neurones of the third order pass to the 

 Cortex. This route is direct and conveys cutaneous and painful sensations. 



(3.) After passing through neurones of the first order by the way of col- 

 laterals to the cells of gray matter of the cord, they travel by the way of the 

 tracts of Gower's (antero-lateral ascending) by neurones of the second order 

 through the dorsal region of the pons and medulla to mid-brain and optic thal- 

 amus. From the optic thalamus neurones of the third order complete the 



