198 



COMPARATIVE ANATOMY 



involutions and irregularities of the surface of the brain, and form 

 a delicate external lamella separating them from the subdural 

 space all along the cerebro- spinal canal. 



In all Vertebrates the subarachnoid space communicates in the 

 region of the hind-brain (medulla oblongata) with the ventricles and 

 central canal, so that there is a free passage into these for the 



albuminous cerebro-spinal or arachnoid 

 fluid, which is mainly formed in connec- 

 tion with the ependyme cells covering 

 the choroid plexuses in the ventricles. 



1. The Spinal Cord. 



The spinal cord is at first of a uni- 

 form diameter throughout, but later, 

 when a richer nerve-supply becomes 

 needed for the extremities, it exhibits in 

 these regions definite swellings the 

 brachial and lumbo-sacral enlargements 

 (Fig. 147). 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 

 it is eventually considerably shorter than 

 the latter. In such cases (e.g. Primates, 

 Cheiroptera, Insectivora, Anura Figs. 

 145 and 147) it passes at its posterior 

 end into a brush-like mass of nerves, the 

 so-called cauda equina ; these lie within 

 the neural canal, and the sacral nerves 

 arise from them. An axial prolongation 

 of the spinal cord nevertheless extends 

 far back, but is reduced to a thin thread- 

 like filum terminate. 



The bilaterally-symmetrical form of 

 the spinal cord is pronounced by the 

 presence of a longitudinal fissure extend- 

 ing along it ventrally ; this ventral fissure 

 is not always present, and the so-called 

 " dorsal fissure," which is formed by 



obliteration of the greater part of the primitive central canal, is 

 due to the presence of a septum formed from connecting sub- 

 stance, the ependyme. 1 If one imagines the points of exit of the 



1 A secondary ventricle-like enlargement of the posterior end of the central 

 canal in many Mammals and other Vertebrates must not be confused with the 

 so-called sinun rhomboidalis in the lumbar region of the cord in Birds, which arises 

 by a separation of the lateral halves of the cord and is filled with modified 

 neuroglia, thus forming a much thickened dorsal septum. 



FIG. 147. 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 longer filum 

 terminale. 



Ce, cauda equina ; F.t, filum 

 terminale ; M .o, medulla 

 oblongata ; Pb, brachial 



plexus 

 nerves 

 plexus 

 nerves. 



PC, cervical 

 PI, lumbo-sacral 

 P.th, thoracic 



