Philosophical Transactions. 225 



columns, we shall find that their surfaces are covered with cineritious 

 matter. If we now clear away the cineritious matter from the 

 columns helow, we shall first discover the two lateral tracts or 

 columns regular as nerves. The columns, when divested of their 

 cineritious matter, are found to be covered with a succession of coats, 

 the superficial layers furnishing the coats of the higher nerves, and 

 the lower layers going off" into the roots of the nerves, as they suc- 

 cessively arise. The sensitive or posterior roots of the spinal nerves 

 disperse in the substance of the lateral columns, and are not derived 

 from the cineritious matter as some assert. Between the lateral 

 columns the cineritious matter lies deep, upon raising it the anterior 

 or motor columns are seen, which resemble the lateral columns. Such 

 are the general features of the spinal marrow. If, returning the 

 parts to their places, we now raise the two posterior columns, we 

 find them diverging at the back of the medulla oblongata, and form- 

 ing the triangular space of the fourth ventricle. Each of these co- 

 lumns is now seen to consist of two, the outermost the larger, and 

 that towards the central line the smaller, and in shape pyramidal. 

 Following them up, they are recognized to be the processus cerebelli 

 ad medullam oblongatam. If now, we trace the cineritious matter 

 on the lateral columns, we can follow it into the 4th ventricle, and, 

 indeed it constitutes one sheet of matter from the cauda equina to the 

 roots of the auditory nerves, and forms a grand septum between the 

 anterior and lateral parts of the spinal maiTow, which belongs to 

 the cerebrum, and the posterior columns which are related to the 

 cerebellum. 



Union of the lateral column:; in the medulla oblongata.— On 

 removing the cineritious matter from the cerebral position of the 

 spinal marrow, the two lateral positions are seen upwards or towards 

 the brain, each of these columns has a double termination, first, in 

 the root of the fifth nerve, and secondly, in the union of the columns, 

 that is, their decussation. These columns lie separate in the spinal 

 marrow ; but at the medulla oblongata, they form one round column 

 rather less than half an inch in length. As it ascends, they are dis- 

 entangled, but do not separate, and they consitute processes of the 

 cerebrum running down from the back of the crura cerebri. 



The septum which divides the right and left sensitive tracts, as 

 seen in the 4th ventricle, and whose nature is still a desideratum, 

 splits to permit the decussation of the columns. When a transverse 

 section is made we observe the motor columns approaching the sen- 

 sitive columns, but no union takes place. 



Observations on the them// qf Respiration. By William Stevens, 

 M. D., &c— This paper commences with stating, that the cause of the 

 dark colour of the venous blood has long been a subject of discussion, 

 and even, at the present moment the question lias not been satisfac- 

 torily decided. We perfectly agree with this affirmation, and cannot 

 discover in what respect Dr. Stevens has assisted in throwing more 

 light upon the subject. We have elsewhere, ( lieeorth, vol. i. 56*) 

 stated the result ol'Vogel, who succeeded in extracting carbonic acid 



VUI.. III. Q 



