152 



COMPARATIVE AISTATOMY 



A differentiation of the primary vascular membrane of the 

 brain and spinal cord into pia mater and arachnoid takes place 

 from the Amphibia onwards, and these two la3-ers become 

 separated in those places where there are deep depressions be- 

 tween the individual parts of the brain ; the deeper of these (pia) 

 adheres closely to the brain, and also penetrates into the ventricles 

 in the form of telm choroidece and plexus choroidei, while the 



superficial one (arachnoid) simply bridges 

 j^ J3 over the depressions (Fig. 122). A lymph- 



sinus (sub-arachnoid space) is thus de- 

 veloped between the two in the Saurop- 

 sida and Mammalia, but this never reaches 

 such an independent differentiation as 

 does the sub-dural (arachnoid) space. 



1. The Spinal Cord. 



The spinal cord is at first of a uniform 

 diameter throughout, but as a richer 

 nerve-supply becomes needed for the 

 extremities, it exhibits in these regions 

 definite swellings — the hrachial and 

 lumbosacral enlargements (Fig. 123). 

 The cord originally extends along the 

 whole length of the neural canal, but 

 its growth is usually less rapid than that 

 of the vertebral axis, so that eventually 

 it is considerably shorter than the latter. 

 In such cases (e.g. Primates, Cheirop- 

 tera, Insectivora, Anura, Figs. 121 and 

 123) it passes at its posterior end 

 into a brush -like mass of lumbo-sacral 

 nerves, the so-called cauda €q%dna, lying 

 within the neural canal. A prolongation 

 of the spinal cord nevertheless extends 

 far back amongst these as a thin thread- 

 like appendage, thafiluin terminale. 



The bilaterally-symmetrical form of 

 the spinal cord is pronounced by the 

 presence of longitudinal fissures running 

 along ifdorsally and ventrally ;i and if one imagines the points of 

 exit of the dorsal and ventral nerve-roots to be respectively con- 

 nected together by a longitudinal line, each half of the spinal 

 cord would thus be divided into three columns, — a dorsal, lateral, 

 and ventral. 



^ The ventral fissure is not always present, and tlie so-called dorsal fissure, 

 "which is formed by obliteration of the greater part of the primitive central canal, 

 is better described as the dorsal septum. 



Fig. 123. — Diagrams of the 

 Spinal Cord and its 

 Nerves. In A the cord 

 passes to the end of the 

 tail, and at B it ends more 

 anteriorly and passes be- 

 hind into a filum termin- 

 ale (F.t). 



M.o, medulla oblongata; Pc, 

 cervical nerves ; Ph, bra- 

 chial nerves ; P.lh, thor- 

 acic nerves ; PI, lumbo- 

 sacral nerves ; Ge, cauda 

 equina. 



