. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. INTERNAL ANA TOMV. 85 The first two pairs of nerves differ in many points from those which succeed them, and are known as the cranial nerves. Thus they have no corresponding ventral roots ; they appear to be exclusively sensory, and do not inner- vate any muscles ; their distribution is confined to the snout, and they are above all characterised by the pres- ence of peripheral ganglionic enlargements which occur chiefly on the finer branches of the nerves near their distal ex- tremities. Furthermore they lie

- Image ID: RE09M7
The Book Worm / Alamy Stock Photo
Image ID: RE09M7
. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. INTERNAL ANA TOMV. 85 The first two pairs of nerves differ in many points from those which succeed them, and are known as the cranial nerves. Thus they have no corresponding ventral roots ; they appear to be exclusively sensory, and do not inner- vate any muscles ; their distribution is confined to the snout, and they are above all characterised by the pres- ence of peripheral ganglionic enlargements which occur chiefly on the finer branches of the nerves near their distal ex- tremities. Furthermore they lie in front of the first myotome. The first pair of dorsal spinal nerves [i.e. the third pair alto- gether) belonging to the first myotome passes from the nerve- tube to the skin through the dissepiment which separates the first myotome from the second, ing the branching of a dorsal spinal AT -.1 n ^1 1- nerve of Amphioxus. (After Hat- And so with all the succeedmg „-i„r,- ^ dorsal roots, they lie at the back of the myotome to which they belong, between it and the next following segment. (Cf. Figs. 2 and 42 A) Shortly after leaving the central nervous system, the dorsal roots divide into two branches, a ramus dorsalis and a ramus ventralis (Fig. 42). These two branches run upwards and downwards respectively, in the gelatinous layer of the sub-epidermic cutis ; that is to say, external to the muscles. In the Craniota the corresponding branches of the spinal nerves lie for the first part of their course internal to the muscles, between the latter and the notochord. The. Diagram illustrat- d.r. Dorsal root. r.d. Ramus dorsalis. ?-.v. Ramus ventralis. r.tv. Ramus visceralis. r.c. Ramus cutaneus ventralis innervating ecto- derm of metapleur. Z'.r. Ventral or motor root, indicated as if in the same plane as the dorsal root.. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these

Similar stock images