THE CEBEBEO-SPINAL AXIS. 663 



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

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

 cerebri, especially towards its posterior extremity, small yellow granules, 

 known as the Pacchionian glands. They are little nuclei of connective 

 tissue that arise from the subarachnoideal tissue ; meningeal granulations 

 would be a better designation for them than that of glands. 



h. The tentorium cerehelli is composed of two lateral laminae, which form 

 a transverse partition between the cerebellum and the posterior extremities 

 of the cerebral lobes. 



Each lamina, 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 

 opening through which the encephalic isthmus passes ; a superior extremity, 

 attached to the parietal protuberance ; an inferior extremity, ^hioh 

 disappears above the Gasserian ganglion, near the fold that surroumls the 

 pituitary gland. 



Of the two faces of these laminsB, the anterior corresponds to the cerebral 

 lobes, the posterior to the cerebellum. 



c. The suprasphenoidal, or pituitary fold, is a thick, slightly salient, and 

 almost circular pad, channeled internally by the cavernous sinus, and 

 circumscribing the sella turcica by enveloping the pituitary gland laterally 

 and posteriorly. 



2. The Arachnoid Membrane. 



The arachnoid presents the same disposition as all the splanchnic serous 

 membranes, in being resolved into two layers — a parietal and a visceral, 

 both constituting a perfectly closed sac, outside 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 lamellre which 

 pass from the pia mater to the dura mater ; around all these its layers form 

 sheaths by becoming continuous with one another. 



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

 face of the parietal layer is united, as we have already seen, to the dura 

 mater. That of the visceral layer covers the nervous axis m spreading 

 itself over the pia mater, but without accompanying it into the anfractuosi- 

 ties of the central mass ; it is beneath this face of the visceral layer that the 

 cerebr-o-spinal (or mharachnoid) fluid is confined m spaces which will be 

 studied hereafter. By their free fa^e, which is smooth and moist, like that 

 of all serous membranes, the arachnoid layers are m contact with each 



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

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

 cranial portion. Everywhere the parietal layer is only composed of a 

 simple layer of epithelium. The arachnoid has no proper vessels or 

 nerves • those which pass through it only accompany each other. 



Spinal Aeachnoid Membrane.— The paneiaZ layer presents nothing of 

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

 its extent bv a somewhat considerable space (the subarachnoid), m wbich 

 the subarachnoid fluid is collected; this space is greatest posteriorly, 

 around the terminal extremity of the cord and the nerves of the cauda 



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



