When is an anatomic entity named "laterale" vs. "lateralis"?
This is actually much more of a lesson in Latin linguistics/grammar.
The root, Lateral, comes ultimately from latus meaning “side&quot; or &quot;flank” in Latin. [Source].

By adding one of these suffixes we instead create an adjective meaning &quot;of the side.&quot;

However, the suffixes differ in their inflection (specifically their gender):
Laterale has the neutral, singular suffix of the nominative 3rd declension.
Lateralis has the masculine, singular suffix of the nominative 3rd declension.



 Source: Wiktionary 

The difference between the gender-assignment of these suffixes is due to Agreement.

[Agreement] happens when a word changes form depending on the other words to which it relates. It is an instance of inflection, and usually involves making the value of some grammatical category (such as gender or person) &quot;agree&quot; between varied words or parts of the sentence.


More specifically, adjectives must agree with the nouns they modify in case, number, and gender.



Let's break down each of these anatomical structures to understand the agreement-induced inflections [Source]:
os cuneiforme laterale - each of these words is neutral/singular:

os = noun, declension 3, nominative, neutral, singular
cuneiforme = adjective, declension 3, neutral, singular
laterale = adjective, declension 3, nominative, neutral, singular

nervus cutaneus antebrachii lateralis - the 2 adjectives match the  masculine/singular noun they describe:

nervus = noun, declension 2, nominative, masculine, singular
cutaneus = adjective, declension 2, nominative, masculine, singular
antebrachii = noun, declension 2, genitive, singular
lateralis = adjective, declension 3, nominative, masculine, singular

 Note: you see a shift to the genitive case for antebrachii because in this instance this phrase means &quot;Lateral cutaneous nerve of the forearm.&quot; 

Latin Linguistics  For Dummies:

Declension = category of nouns in Latin and Greek with broadly similar case formation (i.e., they are inflected/conjugated similarly).

Inflection = the modification of a word to express different grammatical categories such as tense, case, voice, aspect, person, number, gender, and mood.

Nominative = The case used to indicate the subject of a finite verb.

Genitive = The case used to express some relationship such as possession or origin. It corresponds roughly to the English preposition &quot;of.&quot;