304 Anatomy of the Floor of the Fourth Ventricle 
The question that now arises is: what significance have all these 
structures? Retzius makes no attempt himself at explanation. Deje- 
rine, who is acquainted with his descripticn, concludes (op. cit., page 501) 
that the funiculus separans corresponds to the situation of the fasciculus 
solitarius, and that the area postrema forms the ventricular portion 
of the nuclei of the posterior columns. In the trigonum hypoglossi 
he finds only the nucleus cf the hypoglossus. Obersteiner (Nervose 
Centralorgane, 1901, p. 74) about the same time concludes that the 
narrow median zone of this field corresponds to the nucleus funiculi 
teretis. Miss Sabin’s reconstruction of the medulla (Model of Medulla, 
Pons, and Midbrain, Vol. IX, Johns Hopkins Hospital Reports, and 
Atlas .of the Medulla and Midbrain, Baltimore, 1901) shows the struc- 
tures beneath the ventricuiar floor, but the details of their relations with 
surface markings are not especially treated. 
We will now take up in order the study of our series of preparations. 
Typical sections out of the series have been selected, and drawings of 
them, four times enlarged, are reproduced in Plates III and IV. The 
numbers correspond to the levels indicated in Plate II. In the most 
caudal sections (figs. 1, 2, and 3) the fourth ventricle does not appear. 
We see, however, the transition from the spinal cord type of fig. 1 to the 
medulla type of fig. 4. In fig. 1 the posterior longitudinal fissure forms 
a cleft between the nuclei of the posterior columns of the cord, and 
extends ventrally to the grey substance surrounding the central canal. 
In this grey substance antero-laterally there are a few motor cells be- 
longing to the hypoglossal nucleus, which nucleus, however, in sections 
just above this, between fig. 1 and fig. 2, becomes more distinct as a 
compact group of cells. In fig. 2 the posterior longitudinal fissure is 
shorter, owing to the dorsal migration of the central canal. Lying at 
the bottom of this fissure is an area of loose vascular tissue containing 
a few myelinated fibres, and extending ventrally as a wedge into the 
grey substance surrounding the central canal. In this section the grey 
substance can be seen to be differentiated into a darker and a hghter 
area. The former lies ventral, and forms the hypoglossal nucleus. The 
latter lies dorsal to this, and is contiguous with the vascular area in the 
posterior longitudinal fissure. It forms the beginning of the vagus 
nucleus. On one side of the section the fasciculus solitarius can be 
seen lying lateral to the nucleus of the vagus. In fig. 3 the posterior 
longitudinal fissure has become continuous with the central canal, but 
is bridged over dorsally by the obex. The obex has a similar histologi- 
eal structure, and in following through the series is apparently con- 
tinuous with the wedge shaped area described in fig. 2. The vagus and 
hypoglossal nuclei retain the same relations as in the previous sections. 
