( 37 ) 



part of the nucleus a position above the aqueduct is possible. Here, 

 therefore, the cells reach to the medial line without, however, forming- 



Fig. 17. Giconia alba. 



the characteristic medial nucleus as in hydrosaurii and selachii. The 

 cell form is round and vesicular. It is difficult to judge of the exit 

 of the fibres. Some can be seen going from the radix to the motor 

 y nucleus; how the others reach the periphery cannot be stated 

 with certainty from the normal preparations {ciconia, dirysomitls). 



Mammals. Without entering into details regarding each order 

 examined separately, the most striking points of resemblance and 

 difference may be given here. The distinction made by Cajal (1. c.) 

 into 7iucleus infero-posterior and nucl. szipero-anterior within the 

 boundaries of tlie mesenc. V nucleus can be kept for many of the 

 mammals, especially for rodentia, which have been principally exa- 

 mined by Cajal (rabbit, rat). For the two lowest orders of mammals 

 (monotremata and marsupialia) this distinction does not hold good. 

 The part of the nucleus, situated in other mammalia (mono delphia) 

 distally from the mid-brain, is entirely wanting in didelphia or nearl}^ 

 so. [n Echidna It.ystrlx scarcely a single mesenc. V cell is to be found 

 laterally to the aqueduct, still less in the region of the 4''' ventricle. 

 A few have shifted somewhat from the middle line, dorsally to the 

 aq. Sylvii, the large majority lie in every cross plane on the medial 

 line; (cf. fig. 18, plane of the trochlear nucleus). The nucleus is 

 exhausted before the decussation of the nervi trochleares. 



Marsupialia show similar conditions although a vei-y few cells 

 have come to lie already laterallj' to the most rostral part of the 

 4^^> ventricle ; in macropus this is clearer than in didelphys. 



In the case of the last-mentioned animal the nuclei situated dor- 

 sally on both sides distinctly combine to a single one near the medial 



