THE CEREBROSPINAL AXIS. 751 



The anterior extremity advances in a curve to near the optic iotam. 



In aged animals, there are sometimes found on the faces of the falx cerebri, 

 especially towards its posterior extremity, small yellow granulations, known as 

 the Pacchioinaii bodies. They are little nuclei of connective tissue that arise 

 from the sub-arachnoideal tissue ; iiwiuiyeal granulations would be a better 

 designation than that of glands, which is sometimes given to them. 



b. The Tentorium CerebelU is composed of two lateral layers, which form a 

 transverse partition between the cerebellum and the posterior extremities of the 

 cerebral lobes. 



Each layer, coursed internally by one of the transverse sinuses, offers : an 

 adherent convex border, attached to the parieto-temporal crest ; a free concave 

 border, turned inwards and a little forwards, remarkable for. its thickness and 

 solidity, and, with the second lamina, circumscribing an oval 0{x;ning through 

 which the medulla oblongata passes ; a superior extremity, attached to the parietal 

 protuberance ; an inferior extremity, which disappears above tlie (xasserian 

 ganglion, near the fold that sm-rounds the pituitary gland. 



Of the two faces of these layers, the anterior corresponds to the cerebral lobes, 

 the posterior to the cerebellum. 



<:. The supra-splionoidal or pituitary fold, is a thick, slightly salient, and 

 almost circular pad, chaimeled internally by the cavernous sinus, and circum- 

 scribing the sella Turcica by enveloping the pituitary gland laterally and posteriorly. 



2. The Arachnoid Membrane, 



The arachnoid has the same arrangement as all the splanchnic serous 

 membranes, in being composed of two layers — a parietal and a visceral, both 

 constituting a perfectly closed sac, outside of which the cerebro-spinal axis is 

 contained. The cavity of this sac is traversed by the roots of nerves, the vessels 

 of the brain and cord, and filaments and cellular layers which pass from the pia 

 mater to the dura mater ; around all these its layers form sheaths, by becoming 

 contiimous with one another. 



Each of these layers exhibits an adherent and a free face. The adherent 

 face of the parietal layer is attached, as we have already seen, to the dura mater. 

 That of the visceral layer covers the nervous axis, in spreading itself over the 

 pia mater, but without accompanying it into the convolutions of the central 

 mass ; it is beneath this face of the visceral layer that the cerebro-spinal (or 

 sub-arachnoid) fluid is confined in spaces which will be studied hereafter. By 

 their free face, which is smooth and moist, like that of all seroiLS membranes, the 

 arachnoid layers are in contact with each other. 



Structure. — The structure of this membrane resembles that of all others of 

 the same nature. The meshes of elastic fibres are most abundant in the cranial 

 portion. Everywhere the ptirietal layer is only composed of a single layer of 

 epithelium. The arachnoid has no proper vessels or nerves ; those which pass 

 through it only accompany each other. 



Spinal Arachnoid Membrane. — The parietal layer presents nothing of 

 interest. The visceral layer is separated from the spinal cord, throughout its 

 extent, by a somewhat considerable space (the sub-arach)wid), in which the 

 sub-arachnoid fluid is collected ; this space is greatest posteriorly, around the 

 termmal extremity of the cord and the nerves of the cauda equina. 



The adherent face of this membrane is only connected with the external 

 surface of the spinal cord by thin filaments detached from the pia mater. 



