598 MANUAL OF HISTOLOGY. 



arranged in a plexus (Clarke), presenting very fine tubes as a rule, and 

 nuclear elements of the sustentacular substance between the perpendi- 

 cularly descending nerve-fibres. The sustentacular tissue presents itself 

 here in a very pure state, but at the same time, of extreme delicacy. It 

 contains numerous nuclei, and among them some of considerable size, 

 which belong, according to Walter, to small bipolar ganglion cells. 

 Besides this, it presents a layer of large multipolar ganglionic corpuscles, 

 with widely branching protoplasm processes. The whole reminds- us con- 

 siderably of the cortical layer of the cerebellum ( 298). Below, or, more 

 properly speaking, externally, the walls of the bulb acquire a very obscure 

 character from the transformation of the grey matter. Here we encounter, 

 namely, spheroidal pellets of a granular nucleated substance, lying in 

 spongy tissue, from which the peculiar pale peripheral olfactory fibres are 

 given off. These we have already alluded to, and shall consider again in 

 dealing with the organs of smell. 



Thepineal gland, or conarium, is a very puzzling organ, which probably, 

 like cartilage, undergoes early changes. It has been by some supposed to 

 have some relationship to the lymphatic glands (Henle). It presents in 

 a connective-tissue framework, roundish, sometimes complete, sometimes 

 incomplete cavities. These are occupied by two kinds of cellular elements, 

 namely, larger, provided with long, thick, ramifying processes, forming a 

 delicate " reticulum," and smaller, which give off processes in the adult, 

 but not in the infant just born (Bizzozero). 



Numerous peculiar concretions occur here also, known as the acervulus 

 cerebri, a full description of which we reserve for the following section, 

 where we shall consider them in connection with the choroid plexus. 



The hypophysis cerebri has been already disposed of, with the other 

 members of that obscure group of "blood vascular glands" ( 238) to which 

 it belongs. 



In respect to the composition of the brain and spinal cord we have 

 already mentioned all that is necessary to be borne in mind, in the second 

 part of our work ( 190). We have also alluded there to the extremely 

 imperfect state of our knowledge on the subject. 



300. 



We now come finally to the membranes investing the brain and spinal 

 cord, of which there are three. In the first place, there is the dura mater 

 externally, a strong fibrous coat (p. 225) ; then in the middle the arach- 

 noidea, with all the characters of a serous membrane (p. 226) ; and 

 finally, tine pia mater, a delicate internal membrane, immediately in con- 

 tact with the nervous substance of the brain and cord (p. 229). 



The dura mater has been already dealt with, as regards its structure 

 generally, in another chapter. It is rich in elastic fibres. Its relations 

 to the brain and spinal cord are somewhat different. The latter is con- 

 tained within it as in a tube, which hangs down in the spinal canal (lined 

 throughout with periosteum), loose at either side and behind, and only 

 attached anteriorly by a band of connective-tissue to the ligameiitum 

 longitudinale posterius. The matter which occupies the cavity so result- 

 ing is a soft colloid tissue, containing connective-tissue corpuscles and fat 

 cells. Without taking into account the well-known venous plexuses 

 traversing it, the latter is rich in small and extremely delicate blood-vessels. 



Within the cranium, on the contrar} r , the dura mater is intimately con- 

 nected with the periosteum, or, more properly speaking, it plays the part 



