The Spinal Nerves

As was noted above, the dorsal roots of the first two spinal nerves are lacking.

I, II, and III. The ventral branches of these three nerves supply the smaller, ventral neck muscles.

IV. The ventral branch of this nerve innervates with its chief divisions the ventral muscles, the sphincter colli, and the integument of the neck, and sends a small branch to the levator scapulæ superficialis muscle.

V. The ventral branch of this nerve sends branches to the ventral muscles of the neck, to the levator scapulæ superficialis; a large branch goes to the sterno-mastoid; and the rest of the nerve distributes itself in the sphincter colli and the integument and ventral muscles of the neck.

VI. The sixth nerve distributes itself to the ventral musculature and to the integument of the neck, and sends a fairly strong branch to the levator scapulæ superficialis muscle and to the most anterior part of the collo-thoraci-suprascapularis profundus muscle.

VII. The seventh nerve is the first to enter, by a small branch, into the brachial plexus ([Figure 31]). It also sends a branch to the ventral muscles and the integument of the neck, and three branches to various shoulder muscles.

VIII. The ventral branch of the eighth nerve ([Figure 31]) is the second largest nerve of the brachial plexus. It gives some twigs to the ventral muscles and then gives one or two nerves to the collo-thoraci-suprascapularis profundus and the serratus superficialis muscles. The rest of the nerve divides into an inferior and a superior branch which unite with the ninth nerve.

IX. The ninth and tenth nerves are the largest of the brachial plexus. The former, after giving off some twigs to the ventral musculature and to the serratus superficialis and the hinder regions of the collo-thoraci-suprascapularis profundus muscles, unites with the tenth nerve just after giving off the small thoracicus inferior nerve to the costo-coracoideus muscle. After uniting with the tenth nerve the ninth nerve immediately divides into two branches that form loops with branches of the eighth nerve, the whole making a very complicated plexus.

X. The tenth nerve, as noted above, is one of the two largest nerves of the brachial plexus. After giving off a single nerve to the ventral musculature, this nerve unites with the eleventh nerve; it then gives a branch to the costo-coracoideus muscle and forms a loop with the ninth nerve. After giving off a couple of nerves it again divides into two equal branches which unite with similar branches of the eighth nerve.

XI. The eleventh nerve is next to the smallest of the plexus. Besides branches to the trunk musculature it gives a fine twig to the integument of the axilla and unites with the tenth nerve in the brachial plexus. This is the last nerve that enters into the brachial plexus.

Fig. 31. Brachial Plexus of C. Acutus. (From Bronn, after Fürbringer.)

VII-XI.ventralbranches ofseventh toeleventhspinalnerves.
3a.thoracicusanteriorVII.
4.thoracicussuperior V.
7.thoracicussuperiorVI.
7a.proximally-leadingthoracicussuperior.
7b.distally-leadingthoracicussuperiorVIII.
9.thoracicussuperior IX.
10a, 10a₁, 10a₂, 10a₃.thoracicusinferior.
18.cutaneus pectoralis.
19.pectoralis.
21.brachialislongus inferior.
22.coraco-brachialis.
22c.branch for thedistal bellyof bicepsmuscle.
24.muscularbranch forthe humero-antebrachialisinferior.
(25 + 42).cutaneusbrachii andantebrachiimedialis.
29.subscapularis.
31.dorsalis scapulæ(posterior).
32.cutaneus brachiisuperiorlateralis.
33.deltoides inferior.
34.brachialislongus superior.
36.anconæus.
36a.scapulo-humeralisprofundus.

The distribution of the nerves of the brachial plexus is as follows ([Fig. 31]): (a) supracoracoideus to the muscle of that name and to the integument of the breast; (b) thoraci inferiores nerves (10a)—a complex of nerves from the eighth, ninth, and tenth spinal stems—lead to the costo-coracoideus muscles and to the anterior part of the transversus abdominis muscle; (c) the pectoralis (19), a large nerve leading to the muscle of that name; (d) cutaneus pectoralis (18), fine branches from the XIth spinal nerve to the integument of the axilla and the neighboring parts of the breast; (e) coraco-brachialis (22) to the like named muscle; (f) cutaneus brachii et antebrachii medialis (25 + 42) to the medial side of the integument of the upper and fore arm; (g) brachialis longus inferior (21), a large nerve that supplies the biceps and humero-antebrachialis inferior muscles, and then divides into the medianus and ulnaris inferior nerves; (h) subscapularis (29) to the like named muscle; (i) scapulo-humeralis profundus (36a) to the like named muscle; (j) axillaris, a large stem that divides into two main twigs that lead to the skin of the lateral side of the upper arm, to the proximal part of the forearm, to the humero-radialis muscle, and to the deltoides coraco-sternalis muscle; (k) dorsalis scapulæ (posterior) (31) to the deltoideus scapularis muscle; (1) teres major (29b), one (alligator) or two (crocodile) middle-sized nerves to the teres major muscle; (m) latissimi dorsi (29b) to the like named muscle; (n) brachialis longus superior (radialis) (not shown in Figure 31) to the extensor side of forearm and the hand.

Of the spinal nerves between the brachial and crural plexuses Bronn gives no description for the Crocodilia. The most posterior nerve of the former plexus is the eleventh and the most anterior nerve to take part in the latter is the twenty-third, so that there are eleven nerves that are doubtless distributed to the regions not supplied by the two plexuses.

Fig. 32. Crural Plexus and Ischiadic Plexus of the Left Side of A. Mississippiensis. The Nerve Branches are Shown as far as their Entrance into the Muscles. The Crural Plexus is Made up of the Presacral Stems a, b, c. The Obturator Nerve is Built of Two Branches from Stems a & b. (From Bronn, after Gadow.)

a, b, c.presacral nerves.
α & β.postsacral nerves.
s = XXVI.sacral nerve (26th spinal nerve).
2.to extensor ileo-tibialis muscle.
3.to femoro-tibialis muscle.
4.to ileo-fibularis muscle.
5.to ileo-femoralis muscle.
6.to caudi-ileo-femoralis muscle.
7.caudi-femoralis muscle.
8.to flexor tibialis externus muscle.
9.to flexor tibialis internus muscle.
11.to ischio-femoralis muscle.
13.pubo-ischio-femoralis internus.
14.pubo-ischio-femoralis externus muscle.
15.to pubo-ischio-femoralis posterior muscle.

The crural-ischial plexuses ([Fig. 32]) are made up of branches from five nerves, three presacral (a, b, and c), the sacral (s = xxvi), and one postsacral (α); the second postsacral shown in the figure apparently does not enter into the plexus.

The first and second presacrals terminate chiefly in the abdominal and thigh muscles, though the second sends a large branch to fuse with a branch from the third to form the large obturator nerve (N. obt.) that leads to the muscles of the thigh and knee.

The third presacral sends a branch back to fuse with the large sacral (s = xxvi), and these two, together with a branch from the first postsacral, form a complicated network that sends numerous branches to the muscles of the pelvic and femoral regions, to the skin, legs, and tail, as shown in [Figure 32]. The large muscles of the tail are innervated by the regular, metameric nerves of that region, and since there are usually thirty-nine caudal vertebræ, there are probably about that many pairs of caudal nerves, although the last few vertebræ and the muscles of that region are so small it may be that some of the nerves are lacking.