893 



hellar tract, it does not enter the cerebellum along the inferior peduncle. Its 

 fibres pass farther cephalad, through the reticular formation of the pars dorsalis 

 pontis and medulla oblongata, to become reflected dorsocaudad at the level of 

 the isthmus of the hind-brain, and entering the superior medullary velum, proceed 

 with the superior peduncle into the cerebellum. 



The medullary vela are two thin, relatively undeveloped laminae of white sub- 

 stance, representatives of the mid-dorsal wall of the brain tube adjacent to the 

 cerebellar proton, and in the adult brain appear as prolongations of the white 

 central core of the cerebellum. They are the anterior or superior medullary velum 

 { ralvula: or valve of I'ieussens) and the posterior or inferior medullary velum 

 ( velum). 



The superior medullary velum is a thin lamina of white substance spanning the 

 interval between the converging superior peduncles, and with these assisting in 

 the formation of the "roof" of the fourth ventricle. Caudad it is continuous 

 with the white substance of the cerebellum, while on its dorsal surface lie the five 

 to seven folia of the lingula. Cephalad it narrows as the corpora quadrigemina 

 are approached, and a slight median ridge, the frenulum, descends upon the dorsal 

 surface of its apical portion from between the posterior quadrigemina; on either 

 side of the frenulum may be seen the superficial origin of the trochlear nerve. 

 The majority of the fibres in the superior medullary velum are longitudinal; 

 as already described (p. 842), the ventral spinocerebellar (Gowers') tract reaches 

 the cerebellum along the superior medullary velum. 



The inferior medullary velum is a still thinner lamina of white substance which 

 bears the same relations to the nodulus that the superior medullary velum pre- 

 sents to the lingula. Laterad it extends to the flocculus of either side. The 

 inferior medullary velum ends in a free crescentic edge and its ependymal and 

 pial coverings continue as a fused, delicate membrane, the tela choroidea ven- 

 triculi quarti. 



The superior and inferior medullary vela enter the cerebellum at an acute 

 angle, forming the peaked roof (fastigium), while the tent-like recess is called the 

 recessus tecti. 



The Fibres Proper of the Cerebellum. The fibrae propriae of the cerebellum are 

 of two kinds: (1) commissural fibres, which cross the middle line to connect the 

 opposite halves of the cerebellum, some at the anterior part and others at the 

 posterior part of the vermis; (2) association fibres, which are homolateral fibres 

 connecting adjacent lamina? with each other. 



Microscopic Appearance of the Cerebellar Cortex. The cerebellar cortex, on 

 section, presents two marked layers an outer, of a pale gray color, the molecular 

 layer, and an inner, of a rusty-brown tint, the granular layer. At the contact line 

 of these two layers, but more within the molecular than the granular, are found 

 the characteristic nerve elements of the cerebellum, the flask-shaped Purkinjean 

 nerve cells. 



The molecular or ectal layer consists of cells and delicate fibrilla? embedded in a 

 neuroglial network. The cells are small and are characterized by the course of 

 their branching axones which run parallel with the surface of the folium, give 

 off numerous collaterals which pass in a vertical direction toward the cell bodies 

 of the Purkinjean elements and embrace these in a basket-like network. Hence 

 these cells are called basket cells (Fig. 658). 



The Purkinjean cells are flask-shaped, and form a stratum at the junction of 

 the molecular and granular layers, their bases directed toward the latter. Each 

 cell gives off an axone entad, while ectad it gives off numerous dichotomously 

 branching dendrites covering a very large field of the molecular layer. The axone, 

 after giving off several collaterals which pass toward different parts of the granular 

 layer, becomes myelinic not far from the cell body and passes into the white 



