It is with the neural element of muscle tonus that tendon phenomena are intimately associated.
The areas of intervening cortex, arriving at structural completion later than the above sense-spheres, are called by some association-spheres, to indicate the view that they contain the neural mechanisms of reactions (some have said "ideas") associated with the sense perceptions elaborated in the several sensese spheres.
- anp, Anterior neural pore; be, rudiment of buccal skeleton; c, cilia; cb, ciliated band; cc, ciliated groove; cm, cilia at margin of mouth; gl, external opening of club-shaped gland; Hn, Hatschek's nephridium; lm, left metapleur; n, notochord; pp, praeoral pit; ps, primary gill-slits, I, 5, and 13; rm, right metapleur showing through.
The vertebral axis shows a series of broad rings, with distinct neural arches, but no ribs.
1 with the addition of NF, neural fossa protecting the aggregated ganglia of the central nervous system; PVP, left posterior ventral process; PMP, posterior median process.
Snp, Sub-neural arch.
The jerk is obtainable only from muscle fibres possessed of neural tonus.
(Adapted from Hatschek.) pc, Praechordal head-cavity of embryo; cc, collar-cavity (first somite); my, mesodermic somites (myocoelomic or archenteric pouches); ch, notochord with the neural tube (neurochord) lying upon it; np, anterior neuropore; ne, position of posterior neurenteric canal.
Towards the end of the body both neural and haemal arches are continued into forked radial cartilages, which support a median fin.
To these is attached the powerful elastic ligament (ligamentum nuchae, or " paxwax ") which, passing forwards in the middle line of the neck above the neural arches of the cervical vertebrae - to which it is also connected - is attached to the occiput and supports the weight of the head.
C. Labyrinthodonta, with simple biconcave vertebral disks, very slightly pierced by a remnant of the notochord and supporting the loosely articulated neural arch.
Microsauria, nearest the reptiles, with persistent notochord completely surrounded by constricted cylinders on which the neural arch rests.
This plan of structure, apparently evolved out of the rhachitomous type by suppression of the pleurocentra and the downward extension of the neural arch, leads to that characteristic of frogs in which, as development shows, the vertebra is formed wholly or for the greater part by the neural arch (14).
N, Neural canal; ch, chorda.
The work of Camillo Golgi (Pavia, 1885 and onwards) on the minute structure of the nervous system has led to great alteration of doctrine in neural physi nerve cells, that is to say, the fine nerve fibres - since all nerve fibres are nerve cell branches, and all nerve cell branches are nerve fibres - which form a close felt-work in the nervous centres, there combined into a network actually continuous throughout.
The parapodium is represented with its neural or ventral surface uppermost.
The neural laminae are broad, the spines almost obsolete, except in the seventh, and the transverse processes not largely developed.
Embolomeri, with the centra and intercentra equally developed disks, of which there are thus two to each neural arch; these disks perforated in the middle for the passage of the notochord.
Branchiosauria, nearest to the true batrachians; with persistent non-constricted notochord, surrounded by barrel-shaped, bony cylinders formed by the neural arch above and a pair of intercentra below, both these elements taking an equal share in the formation of a transverse process on each side for the support of the rib.
Fins without spines; caudal fin, if present, without expanded hypural, perfectly symmetrical, and supported by the neural and haemal spines of the posterior vertebrae, and by basal bones similar to those supporting the dorsal and anal rays.
Gastrocnemius, tonic contraction obtains; but this, like the last mentioned, is not autochthonous in the muscles themselves; it is indirect and neural, and appears to be maintained reflexly.
A prominent and practically important illustration of neural tonus is given by the skeletal muscles.
The primordium of the neurochord (neural or medullary plate) referred to above becomes closed in from the surface by the overgrowth of surrounding epiblast, and its edges also bend up, meet, and finally fuse to form a tube, the medullary or neural tube.
An important fact to note is that the blastopore is included in this overgrowth of epiblast, so that the neural tube remains for some time in open communication with the archenteron by means of a posterior neurenteric canal.
It is still longer before the neural tube completes its closure in front, exhibiting a small orifice at the surface, the anterior neuropore.