•2«)G ANATOMY OF VERTEBRATES. 



form the sciatic, and into a few filaments passing on to the 

 sacrum, fig. 208, f. 



In all Reptiles there is an anterior and a posterior longitudinal 

 fissure and a central canal dilating into the cpencephalic ventricle. 

 The myelonal canal is surrounded by a thin layer of grey 

 neurine, and, in Lacertians and Crocodilians, it extends as far as 

 the first caudal vertebra : in Ophidians, which have the longest 

 spinal chord, the canal is continued to near the end, which 

 goes as far as the penultimate caudal. The enlargements 

 o-ivino; origin to the nerves of the limbs are best marked 

 in Chelonia, owing to the relative slenderness of the myelon 

 in the trunk. In the Lizards and Crocodiles with nearly 

 equally developed limbs, the more muscular trunk and tail 

 demand a myelonal mass which renders the brachial and iliac 

 enlargements less conspicuous. There are no partial enlargements 

 of the myelon in Serpents ; the nerves, as numerous as the 

 vertebra?, are given off at short and regular distances, as in 

 fig. 188, m. 



§ 54. Membranes of the Myelence-phalon in Ucematocri/a. — Both 

 brain and myelon are immediately invested by a thin but firm and 

 vascular membrane, the outer surface of which, in most Fishes and 

 many Reptiles, bears a stratum of pigment-cells belonging properly 

 to the central layer of the arachnoid, which has here coalesced 

 with the pia mater. This vascular membrane seems, therefore, to 

 be coloured with dark points, and sometimes to be minutely 

 speckled upon a silvery ground ; and the pigmental stratum often 

 accompanies the processes of the pia mater into the ventricles of 

 the brain. There is commonly a remarkable developement of the 

 vascular and pigmental membrane over the fourth, or epencephalic 

 ventricle ; it is largely developed in the Sturgeon, and conceals 

 the rudimental cerebellum in the Lepidosiren. In the Axolotl 

 calcareous particles are superadded to this covering of the epen- 

 cephalon. In Osseous Fishes the commonly considerable space 

 between the brain and cranial walls is occupied by a peculiar loose 

 cellular structure, filled by gelatinous or albuminous fluid, and by 

 oily matter : in the Perch and Bream it seems to consist of an 

 aggregate of minute spherical cells filled with fine colourless 

 oil, the mass being traversed by blood-vessels. Cuvier l found 

 the cells, which he compares to a kind of arachnoid, filled by a 

 compact adipose matter in the Tunny and Sturgeon. This modified 

 arachnoid exists, but in less quantity, in the spinal canal, and 



1 xxiu, i. p. 309. 



