506 OF THE NERVOUS SYSTEM AND ITS ACTIONS. 



is another division, which comes into connexion through the Olivary 

 and Pyramidal bodies, with the anterior and antero-lateral columns. 

 The remaining fibres, which constitute what are improperly called the 

 Crura Cerebri, may be considered as forming two principal tracts, the 

 sensory and the motor ; these being distinguished as such by the cha- 

 racter of the nerves which arise in their course. The sensory tract 

 passes upwards from the posterior columns of the Spinal Cord, and the 

 posterior part of the lateral, to the Thalami Optici ; it is obviously con- 

 tinuous below with the tract in which the posterior roots of the Spinal 

 nerves terminate, and in its upward course it receives the large or sen- 

 sory root of the Fifth pair ; whilst passing through the Pons Varolii, it 

 undergoes a partial decussation. On the other hand,- the motor tract 

 may be regarded as descending from the Corpora Striata and- Tuber- 

 cula Quadrigemina into the anterior and antero-lateral columns of the 

 Spinal Cord ; in its course it gives off the roots of all the motor nerves 

 usually considered as cranial ; and the greater part of its fibres undergo 

 decussation below the Pons Varolii. The functions of the Medulla 

 Oblongata are, therefore, of a double character ; to bring the higher 

 parts of the Encephalon into connexion with the Spinal Cord and the 

 Nerves that issue from it; and to serve as a centre for the reflex 

 movements, performed through the nerves that issue from it. In both 

 respects it corresponds precisely with any segment of the Spinal Cord 

 itself; and there is no reason to believe, that it possesses any other or 

 more special endowments. The importance, however, of the reflex acts 

 of Respiration and Deglutition, over which it presides, causes this por- 

 tion of the Medulla to be the one, whose integrity is most essential to 

 the preservation of life ; and therefore it seems to possess a character 

 more distinctive than it really has. 



Fig. 154. 





^^^^^ 



Dissection of the Medulla Oblongata, to show the connexions of its several strands : A. corpus striatum ; 

 B, thalamus opticus ; c, D, corpora quadrigemina : E, commissure connecting them with the cerebellum; F, 

 corpora restiformia; p, p, pons varolii; st, st, sensory tract; mt, mt, motor tract; a, olivary tract; p, pyra- 

 fifth air 9 ' ary S an & hon '> P> P tic nerve; 3m, root of the third pair (motor) ; 6s, sensory root of the 



895. The chief excitor nerve of the Respiratory movements, as already 

 stated ( 685-687) is the afferent portions of the Par Vagum : but the 



