300 



LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



cyclos tomes except that the intrinsic muscles of the tongue appear to be derived from them. 

 These matters will be clearer after a study of the cranial nerves. (See also Fig. 69.) 



6. The development of the sense organs.— There are three chief sense organs to be consid- 

 ered here— the nose, the eye, and the ear. The nose arises as a pair of evaginations of the ecto- 

 derm of the anterior end of the head. In fishes these persist as a pair of simple olfactory sacs 

 not connected with the mouth. From Amphibia onward the olfactory sacs become connected 

 with the oral cavity by the nasal passages which are then both olfactory and respiratory. 

 The olfactory function is generally limited to the dorsal region of the nasal sac. In most 

 vertebrates the walls of the nasal passages project into the passages as curiously rolled or 

 folded structures, the turb'mals or conchae, which serve to increase the olfactory surface and 

 to warm, strain, and moisten the air passing in. 



The eyes arise in part as evaginations of the diencephalon. The tip of the evagination 

 then invaginates producing a double-walled cup, the optic cup. (See K, Fig. 228, p. 213.) 



otic capsule 



boundary between 

 head and trunk 



spinal nerve 



anterior head 

 cavity 



maxillary 

 mandibular 



pretrematic branch 

 posttrematic branch 



Fig. 69. — Diagram to illustrate the segmentation of the vertebrate head and the relation of the 

 cranial nerves to the segmentation. The numbers above the figure designate the cranial nerves; the 

 numbers in the figure are situated on the head myotomes; the sensory part of the nerves is represented 

 by heavy continuous lines; the motor part by broken lines. The anterior head cavity is the first 

 myotome and therefore the myotome which is numbered 1 is really the second myotome and so on; 

 but as the myotomes were numbered before the anterior head cavity was discovered, the old numbers are 

 generally retained. The myotomes numbered 1, 2, and 3 produce the eye muscles; those numbered 

 4, 5, and 6 degenerate in the majority of vertebrates; those from 7 on probably contribute to the tongue 

 musculature but never form typical parietal muscles such as occur in the trunk. It is seen from the 

 figure tbat the third cranial nerve and the deep ophthahric branch of the fifth belong to the first (really 

 second) head segment; the fourth and remainder of the fifth to the second (third) segment; the sixth 

 and seventh to the third (fourth) segment; the ninth to the fourth (fifth) segment; and the tenth to the 

 fifth to eighth (sixth to ninth) segments. The gill slits are intersegmental in location. The relation 

 of the cranial nerves to the gill slits should also be noted. (After Goodrich in Part IX of Lankester's 

 Treatise of Zoology, courtesy of the Macmillan Company.) 



The internal wall of the optic cup develops into the retina or light-perceiving layer of the 

 eye. The external wall of the cup becomes the pigment layer of the retina. The optic 

 nerve arises in the retina and passes down the stalk of the cup into the brain. The retina 

 is a part of the brain wall, as shown by its manner of formation. The lens of the eye arises 

 from an invagination of the ectoderm over the optic cup. The surface ectoderm then remains 

 as the conjunctiva of the eyeball. The other parts or coats of the eyeball (sclera, cornea, 

 choroid, and iris) are developed from the mesenchyme surrounding the optic cup. 



The vertebrate ear in its complete form consists of three chambers, the internal ear, 

 the middle ear, and the external ear. The internal ear alone is present in fishes. It arises 

 as a saclike invagination of the ectoderm of the head at the level of the hindbrain. This 



