MEDULLA OBLONGATA AND ISTHMUS. 479 



and continues downward on the outer side of the crusta, occupying 

 about one-fifth of its bulk, to the pons, where its fibers also arborize 

 around the nucleus pontis of the same and opposite sides. By 

 means of fibers in the middle peduncle these descending fibers are 

 brought into relation with the cerebellum. 



The fibers constituting the dorsal or tegmental portion of the 

 longitudinal system may be said for convenience to extend from the 

 posterior portion of the medulla and pons to the optic thalamus 

 and cerebrum. They may be subdivided into several tracts : viz., the 

 fillet, the posterior longitudinal bundle, Gowers' tract, etc. 



The filet or lemniscus, consisting of fibers having their origin 

 partly from the cells of the cuneate and gracile nuclei and partly from 

 the cells of the sensor end-nuclei of various sensor cranial nerves, 

 occupies a region in the ventral and mesial portion of the tegmentum 

 throughout its entire extent. Superiorly this mesial fillet divides 

 into two portions, one of which passes to the thalamus and pregem- 

 inum (anterior corpus quadrigeminum), the other to the cortex of 

 the parietal and limbic lobes. The fibers coming from the sensor 

 end-nucleus of the auditory nerve (the lateral fillet) lie on the lateral 

 aspect of the pons and crus. Superiorly they terminate in the post- 

 geminum (the posterior corpus quadrigeminum). 



The posterior longitudinal bundle, an upward extension of the 

 fibers composing a portion of the ground bundle of the spinal cord, 

 is located on either side of the median line just beneath the floor 

 of the fourth ventricle and the aqueduct of Sylvius. As it passes 

 upward collateral branches are given off, some of which arborize 

 around the cell nuclei of the third, fourth, and sixth cranial nerves 

 of the same side, while others cross the median line and arborize 

 around the corresponding cell nuclei of the opposite side. Superi- 

 orly some of the fibers become related to cells in the thalamus and 

 subthalamic region. This bundle of fibers appears to be mainly 

 commissural in character. 



Gowers' tract, the antero-lateral tract of the spinal cord, occupies 

 a position in the lateral region of the formatio reticularis both in the 

 medulla and pons. Continuing upward, it enters the mesial fillet, 

 and in company with it passes through the posterior division of the 

 internal capsule and finally terminates around cells in the cortex of 

 the parietal lobe. 



The transverse fibers of the isthmus are found in the pons. The 

 fibers of the ventral as well as those of the more dorsal regions have 

 their origin in nerve-cells in the cortex of the cerebellum. From 

 their origin they pass through the cerebellar white matter, and through 

 the middle peduncle as far as the median line, where they decussate 

 with fibers coming from the opposite side. Beyond this point they 

 pass to the cerebellar cortex. From their anatomic relations it is prob- 



