THE SPINAL CORD. 131 



The cord is at first of a uniform diameter throughout, but 

 later, in cases where the extremities are well developed and a 

 richer nerve-supply becomes needed, it exhibits in these regions 

 definite swellings the brachial and lumbo -sacral enlarge- 

 ments. The spinal 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 comes eventually to be considerably 

 shorter than the latter. In such cases (Primates, Cheiroptera, 

 Insectivora, Aves, Anura, &c.) it passes at its posterior end into a 

 brush-like mass of nerves, the so-called cauda equina; these lie 

 mostly within the neural canal, and the nerves of the sacral plexus 

 are given off from them. An axial prolongation of the spinal 

 cord nevertheless extends far back, but is reduced to a thin 

 thread-like appendage, the filum terminale. 



The bilaterally-symmetrical form of the spinal cord is pronounced 

 by the presence of longitudinal fissures running along it dorsally 

 and ventrally ; and if one imagines the points of exit of the dorsal 

 (sensory) and ventral (motor) 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 ventral, lateral, and 

 dorsal (anterior, lateral, and posterior columns of human 

 anatomy). 



The external form of the spinal cord in certain Fishes (Orthagoriscus, 

 Trigla), and in embryos of Salainandra atra, as well as its histological struc- 

 ture in the higher Vertebrata, seems to indicate that the unseginented spinal 

 cord of Vertebrates was primitively segmented and paired, and that it has 

 passed in its phylogenesis through a stage which was closely related to the 

 abdominal chain of ganglia of many Invertebrates (e.g. Annelids). A definite 

 segmentation of the mid-brain, cerebllum, and medulla oblongata is also seen 

 in the embryos of all the chief Vertebrate types (Kupifer). 



As regards its minute structure, two parts can always be dis- 

 tinguished in the spinal cord, a white substance, consisting of 

 medullated nerve -fibres, and a gray substance, composed 

 principally of multipolar nerve-cells and non-medullated fibres. 

 Their relative positions to one another vary greatly in the different 

 animal groups, as well as in the different regions of the cord ; the 

 white substance, however, has usually a more peripheral, the gray 

 a more central position. 



The membranes of the spinal cord will be treated of later. 



2. THE BRAIN. 



At a very early stage three swellings may be seen on the anterior 

 enlarged part of the medullary tube, which are spoken of as the 

 primary anterior, middle, and posterior cerebral-vesicles 

 (fore-, mid-, and hind-brain). (Fig. 105, /, //, ///.) 



The cavities of the vesicles, corresponding with the ventricles 

 of the fully-formed brain, are in direct connection with the central 



K 2 



