366 ZOOTOMY. 



longitudinal basilar artery : this passes forwards, sending off branches 

 to tlie medulla oblongata and cerebellum (§ 47S), and just anterior to 

 the pons Varolii (§ 479) bifurcates, forming the paired posterior 

 cerebral arteries ; the internal carotids reach the base of the brain at 

 about the level of the optic chiasma, and each divides into the anterior 

 cerebral artery directed inwards and forwards, and the middle cere- 

 bral directed outwards : the "circle " is completed by the two anterior 

 cerebrals uniting to form a median trunk which passes forwards between 

 the cerebral hemispheres, and by the union of each internal carotid to 

 the corresponding posterior cerebral by a slender posterior com- 

 municating artery. 



477. Tlie medulla oblongata, directly continuous 

 with the spinal cord through the foramen magnum, and 

 resting, when in the skull, on the basi-occipital. It is 

 composed externally of white matter, and is marked on its 

 upper and lower surfaces respectively by the dorsal and 

 ventral fissures, continuous with the similar and similarly 

 named structures in the spinal cord : on each side of the 

 ventral fissure is a narrow longitudinal elevation, the ventral 

 pyramid (Fig. 70,7'./"), immediately external to the anterior 

 part of which is a band of transverse fibres, the corpus tra- 

 pezoideum {c.tz) -. posterior to each corpus trapezoideum, 

 and bounded internally by the ventral pyramid, is an area 

 {o/.i>) answering to the olivary body of the higher mam- 

 malian brain : on each side of the dorsal fissure is the 

 narrow dorsal pyramid (Fig. 71, d.j:), of which at present 

 only a small part is seen (see § 522) : external to this, and 

 forming the dorso-lateral region of the medulla, is the 

 corpus restiforme (r.rs). 



478. The cerebellum {a), on the dorsal side of the 

 medulla oblongata ; a greatly convoluted body, composed 

 externally of grey matter, and consisting of a central lobe 

 or superior vermis (cd^), two lateral lobes {a% and 

 two irregular globular bodies (flocculi, 7?), each of which 



