352 



CHORDATE ANATOMY 



pituitary gland, which is represented by a cluster of vesicles derived from 

 the hypophysis. 



The nourishment of the brain is effected chiefly through the three 

 chorioid plexuses in the roof of diencephalon, mesencephalon, and 

 myelencephalon. 



The spinal cord in cyclostomes has become a thick-walled tube in 

 which three layers are differentiated, an outer marginal layer of fibers, 

 a middle mantle layer of gray matter, and a central ependymal layer 

 which lines the central canal. The gray matter has only two lateral 

 wings or columns instead of the four characteristic of higher vertebrates. 

 A number of giant nerve fibers like those of Amphioxus extend along 



Fig. 311. — Diagram of cranial nerves of lower vertebrate. Eye-muscle nerves 

 omitted; central nervous system dotted, fifth nerve represented as composed of two 

 nerves; lateralis nerves separated from the ninth and tenth nerves. I-X, cranial 

 nerves; 1-5, gill clefts; b, buccalis nerves; c, chorda tympani; g, geniculate ganglion; 

 h, hyoid nerve; i, intestinal (pneumogastric) nerve; J, jugal ganglion, /, lateral-line nerve 

 of X; m, mouth; ind, mandibular nerve; mt, mentalis nerve; tnx, maxillary nerve; op, 

 ophthalmicus profundus nerve; osV, osVII, superficial ophthalmic nerves of V and 

 VII; p, palatine nerve; po, posttrematic nerves; pr, pretrematic nerves; pt, petrosal 

 ganglion; s, semilunar (Gasserian) ganglion; sp, spiracle. (From Kingsley's "Com- 

 parative Anatomy of Vertebrates.") 



the spinal cord, but they do not decussate (i.e., cross to the opposite 

 side of the cord), and they carry impulses caudad only, an indication of 

 the growing dominance of the anterior portion of the nervous system. 

 A primitive trait appears in the location of sensory ganglion cells \vithin 

 the wall of the neural tube. 



The anterior ten pairs of nerves in clycostomes have their exit through 

 foramina in the cranium and hence are known as cranial nerves. It is 

 not unlikely that all correspond to anterior nerves of Amphioxus, e.xcept 

 the optic which is a fiber tract of the brain and not a true peripheral 

 nerve. The ten cranial nerves are the olfactory (I), optic (II), oculo- 

 motor (III), trochlearis (IV), trigeminal (V), abducens (VI), facialis (VII), 

 auditory (VIII), glossopharyngeus (IX), and vagus (X). In lower 

 vertebrates the hypoglossus and spinal accessory nerves are not cranial 

 but spinal. Of the ten cranial nerves, I, II, and VTII are sensory. III, 

 IV, and VI somatic motor, and the others mixed sensorv and motor. 



