152 



Basic Structure of Vertebrates 



gans. It is also the seat of important motor centers concerned with the 

 muscles of the mouth, pharynx, esophagus, stomach, and intestine. 



SPINAL CORD 



Compared to the brain, the spinal cord is simple in external form 

 and relatively simple in its internal structure. Also, its structure is 

 more nearly constant throughout the group of vertebrates. 



In external form, the cord ranges from being dorsoventrally flat- 

 tened to nearly cylindric (Fig. 143). From the medulla backward it 

 tapers slowly, but with a more or less marked brachial enlargement 

 opposite the pectoral region, occasioned by the larger nerves going off 

 to the appendages, and with a similar lumbosacral enlargement 

 opposite the pelvis (Fig. 156). The cavity of the embryonic neural tube 

 persists in the adult cord, but the walls of the tube become so greatly 

 thickened that the lumen appears as an extremely narrow canalis 

 centralis (Figs. 143, 144). The canal is continuous with the ventricles 

 of the brain and occupied by the lymphlike cerebrospinal fluid. The 

 cord is partially divided into symmetric right and left parts by a con- 

 nective-tissue dorsal septum which extends more or less deeply into 



Fig. 143. Cross sections of the spinal cords of various vertebrates. (A) Amphi- 

 oxus. (B) Petromyzon. (C) Squalus. (D) Rana. (E) Alligator. (F) Homo. The mag- 

 nification is not to the same scale. The section of the cord of Amphioxus is enlarged 

 four times as much as that of Petromyzon. In these two animals the axons are non- 

 medullated. The striking differences between the cord of Amphioxus and that of 

 man are bridged over by intermediate conditions in lower vertebrates. (Courtesy, 

 Neal and Band: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



