252 FASCIA AND MUSCLES OF THE HORSE 



downward and forward. The thick belly formed by the convergence of these 

 passes under the triceps to end on the fiat tendon of insertion, which is common 

 to this muscle and the teres major. 



Blood-supply. — Subscapular, intercostal, and lumbar arteries. 



Nerve-supply. — Brachial plexus (eighth cervical and dorsal roots). 



B. Ventral Division 



1. Mastoido-humeralis (AI. brachiocephalicus; levator humeri). — This 

 muscle extends along the side of the neck from the head to the arm. It is 

 incompletely divisible into two portions. 



Origin. — (1) The mastoid process of the petrous temporal bone and the 

 occipital crest ; (2) the wing of the atlas and the transverse processes of the second, 

 third, and fourth cervical vertebrae. 



Insertion. — The deltoid tuberosity and the curved rough line which extends 

 from this to the distal extremity of the humerus. 



Action. — -When the head and neck are fixed, to draw the limb forward, ex- 

 tending the shoulder joint. When the limb is fixed, to extend the head and neck, 

 if the muscles act together; acting separately, to incline the head and neck to 

 the same side. 



Structure. — As already mentioned, the muscle is capable of incomplete 

 division into two parts, the line of division being indicated by the emergence of 

 superficial branches of the ventral divisions of the cervical nerves. The 

 mastoid portion (M. cleido-mastoideus) partly overlaps the other portion (M. 

 cleido-transversarius), which lies dorsal to it. The former is attached to the 

 mastoid process and the occipital bone by a broad tendon which fuses with that 

 of the splenius and trachelo-mastoideus; it is also attached to the tendon of in- 

 sertion of the sterno-cephalicus by aponeurosis. The dorsal portion is attached 

 to the transverse processes by four fleshy digitations. The belly of the muscle 

 is adherent superficially to the cervical fascia and the panniculus, and deeply 

 to the subscapulo-hyoideus. In front of the shoulder its deep face is marked 

 by a tendinous intersection of variable development.^ Here the muscle becomes 

 wider, covers the shoulder joint, passes between the ])rachialis and biceps, and is 

 inserted by means of a wide tendon which it shares with the superficial pectoral 

 muscle. 



Relations. — Superficially, the skin, cervical fascia, the parotid gland, the pan- 

 niculus, brachialis, and branches of the cervical nerves; deeply, the splenius, 

 trachelo-mastoideus, rectus capitis anterior major, omo-hyoideus, serratus mag- 

 nus, anterior deep pectoral and biceps muscles, the inferior cervical artery-, the 

 prescapular lymph-glands, and branches of the cervical nerves. The ventral 

 edge of the muscle forms the dorsal boundary of the jugular furrow. The dorsal 

 border may be in contact with the cervical trapezius, or separated from it by a 

 variable interval. 



Blood-supply. — Inferior cervical, carotid, and vertebral arteries. 



Nerve-supplj/. — Spinal accessory and cervical nerves. 



The pectoral fascia is a thin memlorane covering the surface of the pectoral 

 muscles, to which it is, for the most part, pretty intimately attached. It de- 

 taches a layer which passes between the superficial and deep pectorals. At the 

 posterior edge of the triceps another layer is given off, which passes on the outer 

 surface of this muscle to blend with the scapular fascia; the deeper layer becomes 

 continuous with the subscapular and cervical fasciae. 



The pectoral muscles form a large fleshy mass which occupies the space 



1 This is regarded as a vestige of the clavicle. On this basis the portion of the muscle from 

 the vestige to the arm represents the clavicular part of the deltoid of man. 



