538 THE NEKVOUS SYSTEM. 



transmission to the cerebral cortex, which is concerned with the conscious apprecia- 

 tion of sensations) all impulses of pain, heat, and cold coming from the skin upon 

 the opposite side of the body. The fasciculus spinothalamicus anterior conveys 

 impulses of touch and pressure from the opposite side. 



The spino-cerebellar fasciculi [anterior and posterior] convey to the cerebellum 

 respectively homolateral and bilateral unconscious afferent impulses underlying 

 muscular co-ordination and reflex tone. 



Among the descending tracts that establish connexions between various parts 

 of the brain (see Fig. 475) and the motor nerve-cells in the anterior column may be 

 mentioned the cerebrospinal, the rubro-spinal (from the red nucleus), the tecto- 

 spinal (from the corpora quadrigemina), the vestibule -spinal (from the terminal 

 nucleus of the vestibular nerve), and the bulbo-spinal tracts. The last-mentioned 

 forms a peculiar triangular area upon the surface immediately to the lateral side of 

 the anterior nerve-roots (Fig. 473), but there is great uncertainty as to its mode of 

 origin : it is often called the fasciculus olivospinalis, from the fact that its discoverer. 

 Helweg, believed it to originate from the olivary nucleus in the bulb or medulla 

 oblongata. It may be regarded as an outlying part of the vestibular (or cerebellar^ 

 tract to the motor nuclei of the spinal medulla. 



The fasciculus cerebrospinalis lateralis (O.T. crossed pyramidal tract) is a large 

 well-defined descending tract which lies immediately in front of the posterior column 

 of gray matter, and subjacent to the posterior spino-cerebellar fasciculus, which shut 6 

 it out from the surface. Below the point where the posterior spino-cerebellai 

 fasciculus begins the cerebrospinal fasciculus becomes superficial, and in this 

 position it can be traced as low as the fourth sacral nerve, at which level it ceases t( 

 exist as a distinct strand. The cerebro-spinal fasciculus is composed of an admixture 

 of both large and small fibres. These arise in the brain from the large pyramida 

 cells of the motor or precentral area of the cerebral cortex, and pass downward 

 through various subdivisions of the brain to gain the spinal medulla. As the; 

 enter the spinal medulla they cross the median plane from one side to the othei 

 and it thus happens that the cerebro-spinal tract in the right lateral funiculus of th 

 spinal medulla has its origin in the cortex of the left cerebral hemisphere, and vie 

 versa. As the tract descends in the spinal medulla it gradually diminishes in size 

 and 'this is due to the fact that, as it traverses each spinal segment, numerous fibre 

 leave it to enter the anterior column of gray matter, and end in connexion with th 

 anterior motor cells from which the fibres of the anterior nerve-roots arise. Th 

 entire strand is ultimately exhausted in this way. Numerous collateral fibrils sprin 

 from the cerebro-spinal fibres, and, entering the gray matter, end in a simila 

 manner. In this way a single cerebro-spinal fibre may be connected with severe 

 spinal segments before it finally ends. The lateral cerebro-spinal fasciculus must t 

 regarded as a great motor strand which brings the spinal motor apparatus unde 

 the control of the will. 



Schafer believes that many of the fibres of the cerebro-spinal fasciculus end i 

 connexion with the cells of the nucleus dorsalis. 



In many marsupials, rodents, and ungulates the lateral cerebro-spinal fasciculus li< 

 in the posterior funiculus of the spinal medulla. 



The fasciculus lateralis proprius represents the remainder of the later 

 funiculus. Its fibres are largely derived from the cells situated in all parts < 

 the gray matter, and also from the nerve-cells of the opposite side of the spin 

 medulla. After a course of very varying length in the fasciculus lateralis, the 

 fibres turn medially and re-enter the gray matter. Such fibres may thus 1. 

 regarded as inter-segmental association fibres binding two or more segments of tl 

 spinal medulla together. It may be mentioned that the association fibres whit 

 link together segments of the spinal medulla which are near to each other lie close 

 the gray matter, whilst those which connect the more distant segments are situat- 

 further out in the lateral funiculus. 



Funiculus Anterior. One well-defined tract is situated in the funicul 

 anterior. This is termed the fasciculus cerebrospinalis anterior. The remaind 

 of the funiculus receives the name ,of the fasciculus anterior proprius. 



