THE HIND-BRAIN. 681 



lateral ventricle. In man the original cavity is filled up by neuroglia and its 

 wall becomes thickened, but much more so on its ventral than on its dorsal aspect. 

 Its dorsal part contains a small amount of gray and white matter, but it is scanty 

 and ill defined. A section through the ventral part shows it to consist of the 

 following layers from without inward. (1) A layer of olfactory nerve-fibres, which 

 are the non-medullated axons prolonged from the olfactory cells of the nose, and 

 which reach the bulb by passing through the cribriform plate of the ethmoid bone. 

 At first they cover the bulb, and then penetrate it to end by forming synapses 

 with the dendrons of the mitral cells, presently to be described. (2) Glomerular 

 layer. This contains numerous spheroidal reticulated enlargements, termed 

 glomeruli, which are produced by the branching and arborization of the processes 

 of the olfactory nerve-fibres with the descending dendrite of the mitral cells. (3) 

 Molecular layer. This is formed of a matrix of neuroglia, embedded in which are 

 the mitral cells. These cells are pyramidal in shape, and the basal part of each 

 gives off a thick dendron which descends into the glomerular layer, where it 

 arborizes as indicated above, and others which interlace with similar dendrites of 

 neighboring mitral cells. The axons pass through the next layer into the white 

 matter of the bulb, from which, after becoming bent on themselves at a right 

 angle, they are continued into the olfactory tract. (4) Nerve-fibre layer. This 

 lies next the central core of neuroglia, and its fibres consist of the axons or affer- 

 ent processes of the mitral cells which are passing to the brain ; some efferent 

 fibres are, however, also present, and terminate in the molecular layer, but nothing 

 is known as to their exact origin. 



IV. The Hind-Brain. 



The hind-brain, or epencephalon, comprises those parts which are developed from 

 the fourth cerebral vesicle ; namely, the pons, the cerebellum, and the upper half 

 of the fourth ventricle. 



PONS VAROLII. 



The pons Varolii (tuber annulare) is the bond of union of the various segments 

 of the encephalon, connecting the cerebrum above, the medulla oblongata below, 

 and the cerebellum behind. It is situated above the medulla oblongata, below 

 the crura cerebri, and between the hemispheres of the cerebellum. It is about an 

 inch in length and in thickness, and about an inch and a half in width. It pre- 

 sents four surfaces : superior, which is attached, by direct continuation of fibres, to 

 the mid-brain ; inferior, which is continuous with the medulla oblongata ; while 

 the anterior or ventral and the posterior or dorsal surfaces are free. 



The anterior or ventral surface is very prominent, markedly convex from side 

 to side, and less so from before backward. It consists of transverse white fibres, 

 which arch like a bridge across the middle line, and on either side are gathered 

 together into a compact mass, forming the middle peduncle of the cerebellum. 

 Above and below it presents a well-defined border; below, its transverse fibres 

 slightly overlap the pyramidal bodies of the medulla, which disappear into its sub- 

 stance ; above, the transverse fibres slightly overlap the crura cerebri which emerge 

 from it. This surface rests upon the clivus of the sphenoid bone, and presents in 

 the middle line a longitudinal groove, wider in front than behind, in which rests 

 the basilar artery. 



The posterior or dorsal surface of the pons is free, but is concealed from view 

 by the cerebellum. It forms the upper part of the floor of the fourth ventricle, 

 and will be described with this cavity. 



Structure. Transverse sections of the pons Varolii show that it consists of two 

 parts, which differ in appearance and structure from each other : the anterior or 

 ventral portion consists for the most part of fibres arranged in transverse and 

 longitudinal bundles with a small amount of gray matter ; the posterior or dorsal 

 portion is a continuation of the reticular formation of the medulla, and is called 



