DISSECTION OF THE DOG 209 



Circulus arteriosus [Willisi]. — The anastomoses, mentioned above, 

 result in the production of the arterial circle of Willis, which lies in the depressed 

 area in front of the pons. Anteriorly the circle is formed by the direct or 

 indirect (by the anterior communicating artery) union of the anterior cerebral 

 arteries. Laterally the posterior communicating arteries link the anterior 

 to the posterior part of the circle, which is completed behind by the two 

 terminal branches of the basilar artery. 



Dissection. — Remove the membranes and vessels from the base of the brain. 

 This must be done with the utmost care in order to avoid injury to the 

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

 the several cerebral nerves. The relation of the pia mater to the nerve 

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

 scissors around each root. 



It is well to preface the detailed examination of the brain by a cursory 

 inspection of certain structures at its base. 



The ventral aspect of the medulla oblongata is divided into two latera 

 halves by a median longitudinal fissure (fissura mediana ventralis) continuous 

 with the ventral fissure of the spinal cord and bounded on each side by a pro- 

 minent white strand, the pyramid (pyramis), which disappears under the pons. 

 The median fissure terminates abruptly at the foramen ccecum, a blind depres- 

 sion close to the pons. Lateral to the pyramid, and separated from it by a 

 shallow groove, is the oval facial tubercle (tuberculum faciale). Immediately 

 oral to this is a transverse prominence, the corpus trapezoideum. The prominent 

 pons forms a convex elevation, and is continued laterally into the cerebellum. 

 The broad rounded cerebral peduncles appear oral to the pons and, diverging 

 somewhat, disappear into the cerebral hemispheres. Consequent upon their 

 divergence, a depressed area, the interpeduncular fossa (fossa interpeduncularis) 

 is produced. The oral boundaries of the fossa are formed by the optic tracts 

 — white cords appearing at the edge of the pyriform lobe of the cerebrum — 

 and the optic chiasma produced by the union of the two tracts. The aboral 

 part of the fossa is formed mainly by the posterior perforated substance (sub- 

 stantia perforata posterior), which derives its name from the presence in it of 

 numerous small openings by which blood-vessels reach the deeper brain tissue. 

 The oral portion of the fossa is occupied by the mammillary body and the 

 tuber cinereum. The mammillary body (corpus mamillare) is a prominent 

 white object, frequently showing indications of its double nature, connected 

 with the two columns of the fornix. A grey elevation of somewhat less promin- 

 ence intervenes between the mammillary body and the optic chiasma. This, 

 the tuber cinereum, is connected by means of a hollow stalk, the infundibulum, 

 with the hypophysis, which, as a rule, is left behind in the removal of the brain 

 from the cranium. 



On a level with the interpeduncular fossa, the cerebral hemisphere is raised 



