i5o ORGANIZATION OF THE HEAD v. 8 



dorsal and ventral roots join, but this is not the primitive condition 

 (witness amphioxus and the lampreys), and in the head region the 

 earlier state of affairs is retained, the dorsal and ventral roots remain 

 separate. Presumably the arrangement we find in the head today was 

 laid down in very early times, in the Silurian period or earlier, when 

 the dorsal and ventral roots were still separate. The head, in spite of 

 its specializations, preserves for us a relic of that ancient condition. 



The branchial nerves, such as the glossopharyngeal, show clear 

 signs of this condition. Each has a small pre-trematic branch in front 

 of the slit, a larger post-trematic branch behind it, and a pharyngeal 

 branch to the wall of the pharynx. The pre-trematic branch usually 

 contains mostly sensory fibres from the skin, the pharyngeal branch 

 visceral sensory fibres, including those from taste buds. The post- 

 trematic branch contains both motor and sensory fibres. In addition 

 to these more ventral branches the branchial nerves also usually 

 provide dorsal rami to the skin of the back. 



The three pro-otic somites become completely taken up in the 

 formation of the six extrinsic muscles of the eye, arranged similarly 

 in all gnathostome vertebrates. The four recti roll the eye straight 

 upwards, downwards, forwards, or backwards, and the two obliques, 

 lying farther forward, turn it, as their name suggests, upward or 

 downward and forward (Fig. ioo). Of these muscles the superior, 

 anterior, and inferior rectus and inferior oblique are all derived from 

 the first myotome and are innervated by the oculomotor (third cranial) 

 nerve. The superior oblique, innervated by the trochlear nerve (fourth 

 cranial), is the derivative of the second and the posterior rectus 

 (external rectus of man), innervated by the abducens (sixth cranial), 

 of the third somite. These three nerves are evidently the ventral roots 

 of the three pro-otic somites. At some early stage of vertebrate 

 evolution all the myotomal musculature of the front part of the head 

 became devoted to the movement of the eyes. The muscles originally 

 forming part of the swimming series became attached to a cup-like 

 outgrowth from the brain. 



Most of the rest of the musculature of the head, including that of 

 jaws and branchial arches, is derived from the somatopleure wall of the 

 coelom and is therefore lateral plate or visceral musculature. This 

 lateral plate muscle is indeed better developed in the head than in 

 the trunk, where all the muscles, even of the more ventral parts of the 

 body, are formed by downward tongues from the myotomes. The 

 lateral plate origin of the jaw-muscles at once gives us the clue to 

 the nature of some more of the cranial nerves, the fifth, seventh, ninth, 



