670 A TEXTBOOK OF PHYSIOLOGY 



scattered posterior horn cells, and possibly from cells of Clarke's 

 column. It passes up in the antero-lateral marginal position of the 

 spinal cord, and passes through the bulb and pons, to enter the superior 

 vermis of the cerebellum by the superior peduncle. 



The Spino-Tlialamic Tract consists of a scattered group of fibres 

 lying just internally to the ventral spino-cerebellar tract. Its fibres 

 pass upwards, to end mainry on the same side in the optic thalamus. 

 Some, however, end on both sides in the anterior corpora quadrigemina 

 (10, Fig. 395). 



Tracts which pass from the Brain to the Spinal Cord The Cortico- 

 Spinal or Pyramidal Tract. This tract, consisting of the axons of the 

 large pyramidal cells which exist in the motor region of the cerebral 

 cortex (see p. 7*23) passes down through the brain stem in a ventral 

 position to the base of the bulb, where most of the fibres cross to 

 the other side, forming the motor decussation. These crossed fibres 

 come to occupy within the spinal cord a postero-lateral position 

 (Fig. 397). They gradually terminate during their passage down the 

 cord around the cells at the base of the posterior horn (some say 

 round the cells of the anterior horn). The few uncrossed or direct 

 fibres pass down on the margin of the anterior fissure of the spinal 

 cord ; some end around the cells of the same side, some pass across the 

 anterior commissure, and end around the cells of the opposite side. 



The Rubro- Spinal Tract (or prepyrarnidal tract of Monakow). 

 The fibres of this tract arise from the cells of the opposite red nucleus 

 of the mesencephaloii, and, crossing in the mid-brain (Forel's decussa- 

 tion), pass through the pons and bulb to the spinal cord, occupying 

 therein a somewhat triangular space just anterior to the crossed 

 pyramidal tract (Fig. 397). The fibres terminate around or approximate 

 to the anterior horn cells. 



The Veslibulo- Spinal Tract (antero-lateral descending tract of 

 Loewenthal). This consists of fibres which arise from Deiters' nucleus. , 

 situated in the upper part of the medulla and lower parts of the 

 pons varolii, and pass down into the spinal cord in an antero- 

 lateral position, mingling to a certain extent with the fibres of the 

 ventral spino-cerebellar tract. It constitutes a pathway for impulses 

 which pass from the cerebellum to the spinal cord ; its fibres end by 

 arborizing in the proximity of the anterior horn cells. 



The OUvo-Spinal and Thalamo- Spinal Tracts (tract of Helweg) 

 occupy an antero-lateral position opposite the anterior horn. The 

 fibres pass from the thalamus by way of the inferior olive of the bulb 

 into the cervical region of the spinal cord, Avhere they gradually dis- 

 appear. Their destination is not certainty known. 



Fibres which pass from one Part of the Spinal Cord to Another 

 (Commissural Fibres). These fibres, of which there are many, are not 

 grouped into very definite bundles. Many pass up in the lateral 

 columns, others in the so-called anterior basis bundle, and form 

 connections with the posterior longitudinal bundle of the brain stem ; 

 others lie posteriorly near the postero-median fissure, and form what 

 is known as the septo-marginal tract. 



