528 THE BRAIN OF MAMMALS, BY TH. MEYNERT. 



which formations, like the tuberculum cinereum (fig. 258), 

 project outwardly upon the surface. 



2. The decussation of the pyramids. Although the double 

 nature of the cerebral peduncle stiJl persists within this forma- 

 tion, owing to the presence of the pyramids, the organization 

 is coming to an end by two methods. 



In the first place, the origin of the posterior columns of the 

 spinal cord is completed by the external fasciculi of the 

 pyramids (fig. 260, P 1 ) penetrating internally behind the mid- 

 dle and internal fasciculi of the pyramids, in delicate tracts, 

 extending to the raphe (D), in which they decussate, and as 

 a kind of inferior and posterior system of fibrse arcuatae, 

 curve immediate!} 7 round the circumference of the central canal 

 in order to pass into the posterior column of the spinal cord. 

 This is the superior finely fasciculated, sensory decussation of 

 the pyramids. Clarke, Luys, and Deiters long ago recognized 

 that the posterior columns participate in the decussation of 

 the pyramids. 



The origin of the here prolonged external fasciculi of the 

 crusta from definite parts of the lobes of the cerebrum, has 

 already been discussed at p. 409. Deiters also observes that 

 the fasciculi of the crus cerebri are prolonged without altera- 

 tion of position into the pyramids, and that we must therefore 

 consider the most external fasciculi of the pyramids to be 

 identical with the most external fasciculi of the crusta. 



The .external fasciculi of the pyramids, however, it is obvious, 

 by that decussation, not only pass into the posterior column 

 of the spinal cord, but also enter into connection with 

 the portions of the posterior roots of the first pair of cervical 

 nerves arising in the planes of decussation, so that these 

 nerves may not be excluded from the decussation occurring in 

 the pyramids. 



At the above-mentioned planes of origin, the curves formed by the 

 fasciculi of the posterior columns proceeding from the pyramids are 

 surrounded by the lowermost remains of such fibraB arcuatas (fig. 

 260, AA), as belong to the above -described cerebellar origins of the 

 posterior column, from which they are distinguished by their more 

 posterior position, their connection with the broad region of decussa- 

 tion (D), .their thicker .calibre, and the circumstance that there is no 



