. L'anatomie comparée. Anatomie, comparatif. Le système musculaire 267 dans dotsal et ventrale. Une séparation additionnelle et de déplacements ont suivi l'élargissement de l'optique et les vésicules otiques. De cette manière, finalement, deux ensembles de muscles, une dorsale et une ventrale furent amenés à proximité de l'élargissement de vésicules optiques avec lesquelles ils s'est enfin associé fonctionnellement. Au cours de la transformation des myotomes phylogénétique en muscles oculaires modifications notables de leurs relations nerveuses. Alors que la première myotome conserve tout au long de sa phylogenèse connexion principale avec t

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. Comparative anatomy. Anatomy, Comparative. THE MUSCULAR SYSTEM 267 into dotsal and ventral moieties. Further separation and displacement followed the enlargement of the optic and otic vesicles. In this way, eventually, two sets of muscles, one dorsal and one ventral, were brought into close proximity to the enlarging optic vesicles with which they finally became functionally associated. During the phylogenetic transformation of myotomes into eye muscles noteworthy changes come about in their nerve relations. While the first myotome retains throughout phylogenesis its primary connexion with the oculomotor nerve, the nerve of the second myotome, the trochlearis, acquires a dorsal chiasma and retains connexion only with the dorsal moiety of the myotome which becomes the superior oblique muscle. There is no satisfactory explanation of the appearance of this chiasma. The cells which by their outgrowth produce the fibers of the trochlear HINOBRAIN' '3RD SOMITE. Fig. 221.—The mesodermal somites 1-3 from which the eye muscles develop, as seen in a wax reconstruction of a 5-6 mm. Squalus (elasmobranch) embryo made by Alton F. Chase. The somites are viewed from the left side. The anterior somite of Miss Piatt later breaks up into mesenchyme. nerve are located in the base of the brain posterior to those which form the oculomotor nerve. But the fibers of the trochlear nerve instead of growing directly to the muscle of the same side, as do those of the oculo- motor nerve, grow around the wall of the brain and cross above the brain to connect with the superior obHque muscle of the opposite side. In the days when it was assumed that there is a primary connexion between nerve and muscle, some morphologists thought it necessary to assume that not only the nerves but the muscles had interchanged sides—a sort of internal leap-frog for which there is not a particle of evidence. But even our present knowledge that muscle and nerve become connected with one another secondarily give

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