. Éléments de l'anatomie comparée des vertébrés. Anatomie, comparatif. Comparatif 152 AISTATOMY une différenciation de la membrane vasculaire primaire du cerveau et de la moelle épinière en pie-mère et l'arachnoïde s'effectue depuis le Amphibia, et ces deux la3-ers sont séparés dans les endroits où il y a des dépressions profondes entre les différentes parties du cerveau ; l'approfondissement de ces (PIA) suit de près le cerveau, et pénètre aussi dans les ventricules dans la forme d'choroidei choroidece telm plexus et superficiel, alors que l'un (l'arachnoïde) ponts simplement j^ J3 sur la
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. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. 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 col
