162 THE NERVOUS SYSTEM. 



C. THE COVERINGS OF THE BRAIN AND SPINAL CORD. 



1. The Pia mater closely invests the whole of the brain and 

 spinal cord ; it may,, in fact, be regarded as the flattened,, outermost 

 layer of the connective-tissue material which supports the nervous 

 elements ; fine processes (Fig. 109 c] pass radially from its inner 

 surface to join the general connective-tissue matrix or neuroglia. 

 It is usually pigmented, those portions covering the optic lobes 

 and spinal cord being especially deeply pigmented : on the cerebral 

 hemispheres it contains very little pigment; at times, indeed, 

 in R. temporaria, pigment is absent from this part. 



The pia mater is continued on to the choroid plexuses and pituitary 

 body, and on to the pineal body by means of the pedicle ; the 

 membrane is very vascular, and forms, especially for the cerebral 

 hemispheres and the olfactory lobes, a very important source for 

 the direct supply of blood-vessels. 



2. The Dura mater, or lining membrane of the cranial cavity and 

 vertebral canal, is a connective-tissue membrane containing many 

 very much branched, pigmented cells. This membrane is not so 

 deeply pigmented as the pia mater, except in that portion covering 

 the cerebral hemispheres and the olfactory lobes, which is much 

 darker than the corresponding portion of the pia mater. 



3. The arachnoid membrane is the layer of endothelial cells 

 covering the inner surface of the dura mater : by means of the 

 blood-vessels and nerves, etc., it is continued on to the pia mater 

 of the spinal cord and brain, which it in like manner closely invests. 

 Masses of calcareous crystals are found between the epineurium 

 and the arachnoid (Wiedersheim) on each spinal nerve at its exit 

 from the intervertebral foramen, also on the trigeminal nerves in 

 the cranium. Additional smaller and more irregular masses are 

 found on the dorsal part of the dura mater of the spinal cord. 



D. THE VESSELS OF THE BRAIN AND SPINAL CORD. (Fig. no.) 



The arteries of these organs are derived from the internal carotids 

 and the arteriae vertebrates. As soon as the internal carotid arteries 

 reach the cranial cavity each divides into two branches, an ante- 

 rior (Ramus anterior. Schobl 1 ) and a posterior (Ramus posterior, 

 Schobl) ; the anterior branches course forwards on the lower part of 



1 These names are those adopted by Schobl for corresponding vessels in the newt, 

 the translator has accepted them as being suitable, with slight modification, to the 

 vessels of the fros:. 



