XIX] MUSCULAR SYSTEM 43*7 



development, and the coelom appears from the first as a pair of 

 elongated sacs undivided below, but segmented above. After the 

 complete separation of the dorsal portions as myotomes, the ventral 

 parts of the two sacs unite beneath the intestine, whilst above it 

 their walls become apposed, forming the vertical sheet of tissue 

 known as the mesentery, in which the intestine is slung. 



It is necessary of course for the efficient action of the eyes that 

 they should be movable, and this is brought about by the space 

 around the eyeball becoming converted into a cavity called the 

 orbit, which in the lower Craniata is continuous with the anterior 

 cardinal vein, and thus contains blood (Figs. 214 and 215), but 

 which in the higher Craniata is a lymph space. To each eyeball 

 six muscles are attached, two arising from the anterior part of the 

 orbit and inserted one above and one below the eyeball, and named 

 respectively the superior and inferior oblique; and four arising 

 close together from the posterior corner of the orbit and inserted on 

 the eyeball, one above and one below, the superior and inferior 

 recti, and one antero-laterally, the. internal or anterior rectus, 

 and one postero-laterally, the external or posterior rectus. 



The proboscis cavity and collar cavities of the Hemichorda are 

 represented in the Craniata by two pairs of cavities found in the 

 embryo, in advance of all the myotomes, termed the head- cavities. 

 The anterior of these, termed thepre-mandibular, is joined to its 

 fellow by a very narrow canal running underneath the eyes the 

 pair really constitute a bilobed cavity from whose walls the inferior 

 oblique, superior, inferior and internal recti muscles are developed. 



The collar-cavities are represented by the mandibular 

 cavities, a pair of long, narrow cavities running down the sides 

 of the mouth, joining the splanchnocoel behind and curving up 

 over the eye on each side. From the wall of the dorsal portion 

 of the cavity the superior oblique muscle is derived. The external 

 rectus muscle arises from the first myotome. The muscles derived 

 from the anterior head-cavity are supplied by a common nerve, the 

 third cranial ; the superior oblique is supplied by the fourth, and 

 the external rectus by the sixth cranial. 



Most of the muscles which compress or expand the gill-sacs are 

 derivatives of the wall of the unsegmented ventral portion of the 

 coelom. From the inner wall of this part of the coelom all the 

 muscles of the alimentary canal arise ; these in Craniata are longi- 

 tudinal as well as circular, and the muscles in the walls of the blood- 

 vessels also arise from the coelomic wall. From the myotomes are 



