THE SPINAL CORD. 681 



longitudinal bands which connect the Cerebral mass with the Spinal Cord, but 

 pass through them ; so as in some degree to isolate the two lateral halves 

 from one another, and to form a complete 'septum between the anterior and 

 posterior portions of each. These anterior and posterior tracts of the Crura 

 Cerebri are respectively subservient to the motor and the sensory functions; as 

 is clearly indicated by the endowments of the nerves which are connected 

 with each. 1 The Motor tract is brought into view, by simply raising the super- 

 ficial layer of the Pons, and tracing upwards and downwards the longitudinal 

 fibres which present themselves. It is then found that these fibres may be 

 traced upwards into the*Corpora Striata ; and downwards into the Anterior Pyra- 

 mids and a portion of the Olivary columns ; so that they connect the Corpora 

 Striata with the anterior, and with the anterior portion of the lateral columns 

 of the Spinal Cord. With this tract we find connected passing from below 

 upwards the roots of the Spinal Accessory, the Hypoglossal, the Facial or 

 Portio Dura of the 7th, the 6th or Abducens oculi, the smaller root of the 5th 

 (which can be traced to the part of the Olivary column that passes upwards to 

 the Corpora Quadrigemina), the 4th or Trochlearis (which is attached to the 

 same part of the tract), and the 3d or Oculo-rnotor nerves ; all of which are 

 purely motor in their endowments. The Sensory tract is displayed by opening 

 the Medulla Oblongata on its posterior aspect ; and then separating and turning 

 aside the Restiform columns, so as to bring into view the posterior pyramids. 



[Fig. 181*. 



;y 



SO 



The whole of the pyramidal and olivary bodies and their respective tracts have been removed, and the 

 posterior part of the cord left. D P. Decussation of the pyramidal bodies. D s. Decussation of the cerebral 

 sensory tract, or posterior third of the antero-lateral column, i & c. Inter-cerebral commissure divided. K. 

 Posterior extremity of the thalamus nervi bptici. K A-. Divided end of the same. g. Corpus geniculatum 

 externum. u. Crus cerebri. u u. Divided end of the same, w w. Corpora restiformia. c. Third pair of 

 nerves, c a. Corpora albicantia. e s. Sensory root of the fifth pair. I n. Locus niger. s t. Cerebral sensory 

 tract, p v. Dotted lines marking the situation and width of the pons Varolii, behind which the decussation 

 takes place.] 



1 This was first clearly shown by Sir C. Bell in the "Philos. Transact," 1835. 



