1895. | NEW YORK ACADEMY OF SCIENCES. 233 
1. Superficial layer, a portion of the superficial ventro-ap- 
pendicular muscular sheet, reaching, usually by combination 
with the succeeding layer, a skeletal or fascial insertion in the 
forearm. Usually but few and unimportant traces of this layer 
are found in higher forms, the great bulk of the superficial fibres 
passing from the trunk to the anterior extremity terminating, as 
Pectoralis Major and Latissimus dorsi, respectively on the radial 
and ulnar margin of the humerus. Occasionally, however, the 
Pectoralis Major is not arrested at the radial tubercle of the 
humerus, but, as in Orycteropus, accompanies the Biceps to the 
radius, or, as in the Otter and Wildcat, together with some 
fibres of the Trapezio-Deltoid, accompanies the Brachialis anticus 
to the ulna, or, as in the Seal, expands into the fascia of the fore- 
arm and thus reaches the hand.* 
2. Intermediate layer, constituting the “extrinsic” limb 
muscles of Humphry, derived from the deeper portion of the 
ventro-appendicular sheet, forming muscles arising from the 
pectoral girdle and passing over the elbow to. the bones of the 
forearm. This group is represented by the Biceps, which 
muscle, besides its relation to the Coraco-humerals, presently to 
be considered, presents various degrees of continuity with, and 
separation from, the following layer. 
3. Deep layer, formed by the “intrinsic” limb muscles, aris- 
ing from the humerus, and pursuing the same course as the 2nd 
layer, represented by the Brachialis anticus, or in some in- 
stances (Hippopatamus) by the Brachio-radialis+, which mus- 
cle in this animal is larger and occupies the space on the outer 
side of the humerus from which the Brachialis anticus usually 
arises. 
Here we meet with a replacement of the usual Brachio-ulnar 
by a Brachio-radial flexor, to the complete exclusion of the 
former. 
The 2d and 3d layers, 7. e., Biceps and Brachialis anticus, 
either considered by themselves, or more especially when viewed 
in the light of their mutual relation, present many points of 
morphological interest, and suggest explanations of human re- 
visional variations of the former muscle. 
A general review of the conditions found in the anti-brachial 
flexor group will call for the consideration of the following 
points : 
I. Morphology of the Biceps, the variations of the muscle and 
their significance. 
* Humphry, op. cit., p. 147. 
+ Humphry, op. cit., p. 163. 
