640 NEUROLOGY 



In the horse the posterior part of the falx cerebri does not extend to the corpus callosum, and 

 hence tlie hemispheres are here in contact and adherent to each other over a small area. In the 

 ox and pig the falx does not descend so far as in the horse, while in the dog it completely separates 

 the hemispheres. 



The tentorium cerebelli is a crescentic transverse fold which separates the 

 cerebellum from the posterior poles of the cerebral hemispheres. It is attached 

 above to the tentorium osseum and laterally to the petrosal crests. Its lower 

 border is thin, concave, and free; it forms the upper and lateral boundaries of an 

 opening (Incisura tentorii) which incloses the mid-brain. 



The diaphragma sellae is a small circular fold which roofs over the sella turcica 

 and the pituitary body. It is perforated centrally by an opening (Foramen dia- 

 phragmatis) for the infundibulum. 



The falx cerebelli, a sickle-shaped fold which projects into the median notch between the 

 cerebellar hemispheres in man, is not present in the domesticated animals. There is instead 

 merely a shght thickening of the dura. 



The spinal dura mater (Dura mater spinalis) forms a tube around the spinal 

 cord from the foramen magnum to the second or third segment of the sacrum. It 

 is separated from the periosteum of, the spinal canal by a considerable epidural 

 space (Cavum epidurale) which is occupied by fatty connective tissue and veins. 

 It is held in position chiefly by the sheaths which it furnishes to the roots of the 

 spinal nerves, and in its anterior part by two ligaments; the latter connect it with 

 the inferior atlanto-occipital membrane and with the odontoid process. It is large 

 in proportion to its contents, but its diameter is not tmiform. It is largest in the 

 atlas, small in the thoracic region, and l^ecomes very small in its terminal part, 

 where it incloses the delicate filinn terminale of the spinal cord. 



The subdural space (Cavum subdurale) is the cavity between the inner surface 

 of the dura mater antl the arachnoidea. It is a mere capillary space which contains 

 just sufficient fluid to moisten its surfaces; this fluid is usually regarded as lymph, 

 which is replenished by filtration through the walls of the blood-vessels. The 

 space is in communication with the lymph-paths of the nerve-sheaths. 



THE ARACHNOIDEA 



The arachnoidea is a very delicate and transparent membrane situated between 

 the dura and pia mater. Its outer surface forms the inner wall of the subdural 

 space and is covered by a layer of endothelium continuous with that of the opposed 

 surface of the dura mater. Between it and the pia mater is the subarachnoid space 

 (Cavum subarachnoideale) which contains the cerebrospinal fluid. An inner sur- 

 face can scarcely be said to exist, since deeply the membrane becomes a reticulum 

 of fine fibers which traverse the subarachnoid space and are attached to the pia 

 mater. The arachnoidea furnishes sheaths to the cerel'jral nerves from their super- 

 ficial origins to a variable but usually short distance beyond the emergence from 

 the dursal sac. In the case of the optic nerve this sheath extends to the eyeball. 



The arachnoidea of the brain (Arachnoidea encephali), except in the case of 

 the great longitudinal and transverse fissures, does not dip into the stilci on the 

 surface of the brain. On the summits of the gyri it is so closely attached to the pia 

 mater that the two form practically a single membrane. Its outer part bridges 

 over the sulci, and here the subarachnoid space is partially divided up by the loose 

 arachnoid tissue into intercommimicating cavities. In certain situations the 

 arachnoidea is se])arated from the pia by spaces of consideral)le depth and extent. 

 These enlargements of the subarachnoid space are termed subarachnoid cisterns 

 (Cisternae subarachnoidales) . In them the subarachnoid tissue does not form a 

 close network, but consists of a relatively small mmiber of long, thread-like strands 

 which traverse the cavity. 



