DISSECTION OF THE DOG 195 



opposed surfaces of the two membranes are smooth and glistening, and 

 moistened by a small quantity of a serous fluid. 



Falx cerebbi. — The falx cerebri is one of the folds of the supporting layer 

 of the dura mater. It lies in the median plane between the two cerebral 

 hemispheres, and in form resembles a sickle with a gentle curve. The convex 

 border is attached to the middle line of the cranial vault from the crista galli 

 to the osseous tentorium, and contains the sagittal sinus. The concave border 

 is inferior and free. 



Dissection. — The falx cerebri should now be cut across as close as possible 

 to its anterior end, and turned backwards. When this has been done, 

 veins from the interior of the brain will be noted joining the system of 

 blood-sinuses at the tentorium. The deep veins join to form a straight 

 sinus and this, in its turn, joins the sagittal sinus. The converging point 

 of the sinuses is known as their confluence (confluens sinuum). 



The brain must now be removed. Begin by cutting across the medulla 

 oblongata on a level with the foramen magnum. At the same level cut 

 the basilar artery, which will be found immediately ventral to the medulla, 

 and the spinal roots of the accessory nerves, which run lateral to the 

 medulla. Now introduce the handle of the knife under the medulla 

 and gently raise the brain from the base of the cranium. If the head 

 be placed in a vertical position, with the nose resting on the table, the 

 removal of the brain will be facilitated. The nerves and vessels asso- 

 ciated with the base of the brain are thus put on the stretch, and should 

 be severed one by one. In cutting the cerebral nerves it is well to divide 

 them close to the dura on the one side, and close to the brain on the 

 other. The last eight nerves are fairly close together, but may be dis- 

 tinguished by differences in size and mode of exit from the cranium. 

 The twelfth nerve alone leaves by the hypoglossal foramen. Then 

 comes a group formed by the eleventh, tenth, and ninth, which leave 

 by the jugular foramen. The eighth and seventh nerves enter the 

 internal acoustic meatus in company. The sixth, a small nerve, and 

 the fifth, the largest of the cerebral nerves, pierce the dura mater close 

 together at the medial border of the temporal bone. At this stage 

 notice that a small lobule of the cerebellum is lodged in a depression 

 of the temporal bone. A little care is necessary to remove the lobule 

 without injury. 



The fourth nerve is the smallest of all the cerebral nerves and appears 

 round the side of the cerebral peduncle. The third nerve is much larger. 

 The next structure interfering with the removal of the brain is the infundi- 

 bulum, a hollow, median connection between the tuber cinereum and 

 the hypophysis. Just in front of the infundibulum, and only a little 

 distance from the median plane, are the two internal carotid arteries. 



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