Motor System — Muscles and Skeleton 



117 



Fig. 115. Diagrams showing the development of the primordial skull. Ventral 

 aspect. (A) Early stage, before the appearance of cartilage. The notochord is seen 

 lying along the nerve-cord as far forward as the hypophysis. The three sense-organs, 

 nasal sac, eye, and ear, have already appeared. (B) This stage shows the trabeculae 

 (/), the parachordals (p), and the cartilaginous capsules around the sense-organs. 

 (C) In this the trabeculae, the parachordals, and the nasal and otic capsules have 

 fused into a single mass, the primordial skull, or chondrocranium. The anterior 

 end of the notochord is embedded in it. The cartilaginous capsule of the eye 

 remains free to allow the necessary movements of the eyeball. (Courtesy, Wilder: 

 "History of the Human Body." New York, Henry Holt & Co., Inc.) 



The eyes are not enclosed within the cranium. The essential nerv- 

 ous part of the eye, the retinal layer of the eyeball, is developed by 

 outgrowth from the middle region of the brain. This outgrowth occurs 

 before the cranium begins to develop. The prospective retinal material 

 acquires the form of a hollow hemisphere, and this "optic cup" (Fig. 

 115A) is connected with the brain by a slender optic stalk. The optic 

 cup becomes surrounded by embryonic tissue (derived from the middle 

 embryonic layer, the mesoderm) which is potentially skeletogenous. It 

 builds around the optic cup a supporting and protective layer, the 

 sclera (Fig. 115B, C), which in most vertebrates is to a greater or less 

 extent cartilaginous, and in many reptiles and birds the cartilage may 

 even ossify to form small bony plates (sclerotic bones — Fig. 369). 

 In most mammals, however, the sclera consists only of dense fibrous 

 connective tissue. The sclera, especially when it is cartilaginous, re- 

 sembles the capsules of the olfactory and otic sacs. It differs from them 

 in that it does not become joined to the brain-case — a quite intelligible 

 difference, because the eyeball must be free to move. The eyeball does, 

 however, derive much protection from the cranium because it lies in a 

 lateral recess, the orbit, between the laterally projecting nasal capsule 

 in front and the otic capsule behind. 



