188 THE CEREBRAL HEMISPHERES. 



a considerable interval between the cerebellum and the back of the medulla 

 oblongata (fig. 131). In the spinal canal, where it surrounds the cord, it is of 

 considerable extent. It is occupied, in both brain and cord, by trabeculse and thin 

 membranous extensions of delicate connective tissue, connected on the one hand 

 with the arachnoid, and on the other with the pia mater. This tissue is most 

 abundant where the space between the two membranes is least. It is dense in the 

 neighbourhood of the vessels, and is continuous with the tissue of their walls. In 

 several places therefore the arachnoid is separated by larger intervals than at other 

 parts from the pia mater. 



The spaces which are thereby produced are termed cisterna arachnoid ales (reservoirs of 

 subarachnoid fluid). They have been studied especially by Key and Retzius, and by Duret. 

 The principal are situated at the base of the brain. The largest (cistema cerefallo-medullaris) 

 lies between the middle part of the cerebellum and the medulla oblongata, and is directly 

 continued from the subarachnoid space of the cord. Others lie in front and at the sides of 

 the medulla oblongata and pons (c. pontis media s. basilari* and c.jnint-tn lateralis'), below the 

 interpeduncular space (c. interpeduncularis'), over the cerebral peduncles (t\c.peripedunculares), 

 behind the optic chiasma (c. cJviasmatis), in front of the chiasma (c. lamina? cinerete), in the 

 fossae Sylvii (c.c. fossa Sylvii), and over the corpus callosum (c. corporis callosi). These are 

 all in free communication with one another, being only partly separated by imperfect septa of 

 arachnoid tissue. They receive the subarachnoid clefts (flumlna) which follow the course of 

 the great fissures (Rolandic, Sylvian, parallel, &c.), and which themselves receive the clefts 

 which follow the course of the secondary and tertiary fissures (rivi and riruU of Duret). 



The subarachnoid space communicates with the ventricles of the brain by means 

 of the foramen of Magendie (fig. 131, fM~), an opening into the lower part of the 

 fourth ventricle, through the pia-matral expansion (tela choroidea inferior) which 

 covers the ventricle ; through apertures in the lateral recesses, one on each side, 

 behind the upper roots of the glossopharyngeal nerve, in the pouch-like extension 

 of the membrane beneath the flocculus ; and perhaps also at the clefts described by 

 Merkel in the descending cornua of the lateral ventricles (see p. 126). 



The cerebro-spinal fluid is lodged in the subarachnoid space in the meshes of 

 the trabecular tissue, and since this space communicates with the ventricles of the 

 brain, the fluid within these must be regarded as of the same nature. It differs 

 in many respects from ordinary lymph (compare Halliburton, Cerebrospinal Fluid, 

 Journal of Physiology, Vol. X.). 



The spinal subarachnoid space (fig. 132, k, I) is divided by an imperfect fibrous 

 septum on either side termed the ligamentum denticulatum (g) into anterior and 

 posterior portions. As was pointed out by Magendie there also exist a sort of septum 

 dividing the subarachnoid space at the back of the cord (septum posticum) (c), the 

 relations of which have been carefully studied by Axel Key and G. Eetzius. It is a thin 

 membranous partition, which passes in the median plane from the pia mater covering 

 the posterior median fissure of the cord to the opposite part of the loose portion of 

 the arachnoid membrane. It is most perfect in the cervical region, being incomplete 

 below. It consists of numerous fine lamellae, enclosing between them small spaces, 

 within which run the larger blood-vessels. Subarachnoid trabeculse also connect the 

 nerve-roots with the inner surface of the arachnoid, and in the dorsal region fine 

 membranous trabeculse extend between the posterior nerve-roots and the posterior 

 septum. In most parts however the subarachnoid trabeculse are far less developed in 

 the spinal canal than in the cranium. 



The nerves as they pass from the brain and spinal cord receive their perineural 

 covering from the pia mater, and, in addition, two looser sheaths, an outer from the 

 dura mater, and an inner from the arachnoid (fig. 128). Upon the optic nerve 

 these sheaths remain distinct and separate, so that the space which each encloses 

 may be injected, the outer from the subdural, the inner from the sub-arachnoid space. 

 On the other nerves the arachnoidal sheath soon ceases, and the single sheath eventually 



