NERVOUS SYSTEM. 313 



triangular figure as it extends backward, and terminates with the 

 falx, at the cerebral process, where the lateral sinuses and it 

 conjoin their canals. Internally, this sinus presents to view an 

 uneven surface, upon which the veins are seen terminating by 

 large open orifices discharging their blood in the same direc- 

 tion that the stream takes within the sinus itself. Here and 

 there are seen slender chords, crossing the canal from side to 

 side ; which serve to strengthen its parietes and prevent over- 

 distention. 



The two lateral sinuses are formed within the duplica- 

 ture of that part of the tentorium which is attached to the tem- 

 poral and occipital bones; one extending to the right side, the 

 other to the left side. They begin at the cerebral process, where 

 the longitudinal sinus ends ; and they terminate at the foramina 

 lacera basis cranii in the jugular veins. Their canals are also tri- 

 angular, and are intersected by fibrous filaments. They receive 

 veins both from the cerebrum and cerebellum. 



The cavernous sinuses, so named from the cavernous 

 appearance of their interior, which includes a structure having 

 some resemblance to the corpus cavernosum penis, are formed 

 out of the dura mater upon the body of the os sphenoides. 

 These cavities are remarkable from receiving some important 

 nervous trunks in their passage from the brain, and from lodo-- 

 ing the terminations of the internal carotid arteries. They com- 

 monly communicate with the 



Sub-occipital sinuses. — These are likewise of the same 

 formation, and are found upon the cuneiform process of the os 

 occipitis, running longitudinally to the foramen magnum. They 

 receive veins from the cerebellum and posterior parts of the 

 cerebrum. 



THE PIA MATER is the membrane immediately investing 

 the brain, sending processes into its substance which separate 

 the cortical portions of its convolutions, and having a further 

 connexion with the organ through the medium of the numerous 

 interpassing bloodvessels. The reticular arterial ramification we 

 descry upon it, is for the purpose of transmitting vessels of mi- 

 nute dimensions into the interior of the brain; which, thouoh 

 they can be traced by us no deeper than the cortical, are ulti- 

 mately destined for the cerebral substance. The pia mater not 

 only clothes the convoluted part of the cerebral mass, passing 

 from one hemisphere to the other, but it gains admission into 

 the ventricles, and gives them a lining. It differs altogether in its 

 aspect and structure from the dura mater; presenting a smooth 

 surface exteriorly, but a rough and villous one next to the brain, 

 and being composed of a beautiful network of bloodvessels, 



