176 TOPOGRAPHICAL ANATOMY OF THE 



Circulus arteriosufi. — The anastomoses of the chief cerebral arteries 

 results in the production of a polygonal arterial circle (of Willis), which 

 lies in the depressed area in front of the pons, and surrounds the 

 hypophysis and optic chiasma. Anteriorly the circle is closed by the 

 union of the two anterior cerebral arteries. Laterally the posterior 

 communicating arteries link the anterior to the posterior part of the 

 circle by establishing a connection between the internal carotid and 

 posterior cerebral arteries. The circle is completed behind by the 

 diverging posterior cerebral arteries. 



Dissection. — Remove the vessels and pia mater from the base of the 

 brain. This must be done with the utmost care to avoid injury to the 

 brain tissue generally, but especially to prevent tearing away the roots of 

 the several cerebral nerves. The pia mater forms sheaths to the various 

 nerve roots. The relation of the membrane to the nerves is consequently 

 so intimate that it is probably safest to cut the membrane with scissors 

 around each root. 



Before proceeding with a detailed examination of the brain, the 

 dissector should amplify somewhat the brief general survey already 

 made of the structures at its base. 



A ventral median fissure, continuous with the ventral fissure of 

 the spinal cord, divides the medulla oblongata into two lateral halves, 

 and terminates abruptly at the foramen coiCU7n, a blind depression close 

 to the pons. The fissure is bounded on each side by a prominent white 

 strand, the jpyramid, which disappears under the pons. Lateral to the 

 pyramid, and separated from it by a shallow groove, is the flattened, 

 oval facial tubercle. Immediately in front of this is the transverse 

 prominence, the corpus trapezoid eum.^ 



The salient pons forms a convex transverse elevation that can be 



traced laterally into the hemisphere of the cerebellum. The broad and 



rounded cerebral peduncles appear from underoeath the anterior 



boi'der of the pons and, diverging somewhat, disappear into the cerebral 



hemisphere dorsal to the optic tracts and chiasma. The divergence of 



the peduncles produces the interpeduncular fossa, the anterior boundary 



of which is formed by the optic tracts — definite white cords that appear 



between the piriform lobe of the cerebrum and the cerebral peduncle — 



and the optic chiasma that results from the union of the tracts. The 



posterior part of the interpeduncular fossa is formed mainly by the 



posterior perforated substance, which derives its name from the 



numerous small openings by which blood vessels gain the deeper brain 



tissue. The anterior part of the fossa is occupied by the mammillary ^ 



body and the tuber cinereumj^ The mammillary body is a prominent 



^ Corpus [L.], body. Tpaire^oeiSrjs (trapezctides) [Gr.] ; trajieziura-shaped [Tpaire^iov 

 (trapezion), a small table or counter). 



2 Mammilla (dim. of mamma) [L.], a nipple. ^ Cinerens [L.], ashy, grey. 



