362 COMPARATIVE ZOOLOGY OF CHORDATES 



rise to the superior oblique ; and the hyoid somite, also growing 

 forwards, becomes attached to the posterior part of the eyeball, 

 forming the external rectus muscle. 



In higher vertebrates, the prootic somites are not always 

 separately recognisable as such, and in these cases the eye- 

 muscles appear to arise from masses of mesenchymatous 

 mesoderm-cells. In others, the manner of development is 

 the same as that described for the dogfish. 



In the lower vertebrates, the head has no greater mobility 

 than any other part of the body ; indeed, in several bony fish 

 it has none, for the back of the skull is connected with the 

 pectoral girdle by a chain of bones. The neck has not yet 

 evolved in these animals. The neck is a region of flexibility 

 which enables the head to be moved without moving the body. 

 This is made possible by the specialisation of the most anterior 

 vertebrae. In reptiles, the differentiation of the first two 

 vertebrae into the atlas and axis appears, and the head is then 

 able to hinge on the transverse axis (as in signing " yes"), and 

 on the longitudinal axis (as in signing " no "). Movement of 

 the head to the side is effected by the flexibility of the next 

 posterior vertebrae, the ribs of which do not get attached to 

 the sternum. In this way, the cervical vertebrae differ from 

 the thoracic. When the neck is very long and capable 

 of extensive twisting, it is common to find vertebrarterial canals, 

 formed between the centra and the ribs fused on to them. 

 These canals protect the artery from being kinked when the 

 neck is twisted. In mammals, the neck contains seven 

 vertebrae, except in three species only. 



Literature 



Goodrich, E. S. " Proboscis Pores " in Craniate Vertebrates. Quarterly- 

 Journal of Microscopical Science, vol. 62, 1917. 



On the development of the Segments of the Head in Scyllium. 



Quarterly Journal of Microscopical Science, vol. 63, 1918. 



van Wijhe^J. W. Ueber die Mesodermsegmente und die Entwicklung der 

 Nerven des Selachierkopfes. de Waal, Groningen, 1915. 



